Do they ecsist? I only can find long ones, but I need a compact one like they have for PL-C lights. Don't know if PL-C  electronic ballast works good on a T8 tube light...
Does anybody knows? 

PL-L lamps are same as T8. PL-C are for the most part lower current lamps

The ballasts for PL-L generally exist in compact form, but in the VS datasheets (first thing i have on hand) i can only find the non dimmable ones, looks like they make the dimmable in compact form only for PL-C/PL-T

If you are looking for PL-C/PL-T, the arc voltage gotta match. It is fairly close with the 32W PL-T. However, it might underpower the T8 lamp somewhat or inappropriately heat the electrodes, leading to short lamp life, esp when dimmed. Does not look like great solution to me

Thanks for your answer. I was also thinking of that solution with the PL-C / PL-T ballast to use it for T8 lights. But I must sell my lights to other people so I must be sure that it works good, also after a year or so..  I've in my living room an Osram qti intelligent 2x36w ballast running on T12 Philips 40w/32 lights. But it gives some problems when dimming sometimes. It will be fluctuating up and down, I think because of the problems what you say in your message. I don't must have such problems if I sell my lights with 2 TL-D 36w lights running on a PL-T evsa. 

Definitely not good for a commercial product. Is there really a problem to find a way to fit in "long" ballast ?

Yes there is. They must be bult in this kind of fixtures;
http://www.lighting-gallery.net/gallery/displayimage.php?album=3891&pos=1&pid=116317
The ballasts are in one of the aluminium ends of the fixture. I don't want it visible because of all the glass. There is only room for a compact electronic ballast.

Explosion proof lighting in hazardous location does not need to be dimmable, so i guess they are used in an "artistical" way in a non hazardous environment. Then how about ballast in an external box ? Either hidden or made to appear as electrical box of some sort as part of the artwork, connected to the lantern. For 2x36W you'd need 7 wires between box and lantern (6 current carrying + 1 Earth for the enclosure), many control cables are ok for this

Then how about ballast in an external box ?

Practically all the dimmable ballasts are all high frequency devices and there the wire length between the lamp and ballast is very limited and in fact do not allow to place the ballast otherwise than directly next to the lamp into the fixture.
Not observing this limitation means you violate the electromagnetic emission limits (the wires form an antenna). Next problem is the parasitic capacitances interfering with the ballast functionality, mainly at low light dimming levels and the EOL protection.

So remote HF ballast really is not any good idea at all.

Do they ecsist? I only can find long ones, but I need a compact one like they have for PL-C lights. Don't know if PL-C  electronic ballast works good on a T8 tube light...
Does anybody knows? 

If the tube is rated for similar current and has similar arc drop, it may work. MAny of those tubes are 0.22A (the 13 to ~20W if I remember well) and these will work with F15T8 or so.
The only problem is, you have to figure out, which filament connection is the "hot" one in terms of the presence of the HF voltage towards ground, as with linear tube you need one set of wires to be long and these may be just the "cold" end ones (the PL-C ballasts are not designed to work even with the wires of the tube length on the hot end)

Thanks for your ideas. But what Ash says is right. The wires and ballast must be near the lamp. It's not really a good choice what I do to put the ballast in the aluminium end also, behind the lamps. But the wires are next to the lamps, going to the other site of the electrodes and that's very important. Specially with dimming. Hopefully that's good enough but I see it if it works when I've done it
I think I just must try it with this one...
http://www.osram.com/osram_com/products/electronics/electronic-control-gears-for-traditional-lighting/ecg-for-fluorescent-and-compact-fluorescent-lamps/ecg-for-fl-and-cfl%2c-dimmable-dali/quicktronic-intelligent-dali-dim-cfl/index.jsp?productId=ZMP_56472

Thats what Medved said. However, it depends on the length of wire. If you place the box right besides the lantern, the "hot" wires won't  be any longer than they are in some factory made lanterns with internal ballast

With the PL ballasts there is problewm specifdic for dimming : The ballast heats the electrodes on a separate supply when it is dimmed. The supply for PL electrodes may not be appropriate for T8 electrodes

How about T5 lamp in place of the T8 ? Can you find a ballast for that ? Alsao T5 are low current too, so you have better chance to find one that matches the specs of a PL atleast approximately

Oh yes, sorry, I've switched your names and I saw an other text that I've missed from Medved because I was also typing a new text that time..
About the T5 option. I don't like the look of the slim T5 in that fixture and I must change the lamp holders also. I like it most with T12 lights but that's not a good option anymore. So T8 is now my best choice.
About the different filament  of T8 and PL electrodes I think you're right.  But I saw the same fixture on the internet where they've used 1-10v ballast that must be a compact one.  So I think they've used a PL-C or T ballast for it.
But how do I know what is the 'hot and cold' site? I like to place the ballast in the aluminium site behind the lamps. And I will see how it works and how the filaments react. There is no room in the glass tube near the lamps.

What about T5 in adapters (so use same lamp holders) + T12 diffused sleeve ?

In many ballasts the hot side is indicated, sometmes like "keep wires 3 4 short". Sometimes not on the ballast itself but in the manual. If not, there still are ways you can try :

 - Check the documentation for Emergency modules from the same manufacturer. Many times the circuit how to connect the module to the ballast is present, and you can figure out which side is hot from that

 - Measure the field around the wires with a non contact voltage tester

 - Peek inside the ballast

The ballasts in the lanterns you seen might have been custom order, and might have been not the best choice for the task anyway - I seen manufacturers use wrong ballasts at times