The starters with the little push buttons on top I think were made for places with a large number of fixtures so that if bulb would not start, the starter would lock out the bulb instead of keep trying to light.
For most members here that watch their lighting very close, the lock out starters would not be much of a advantage over normal starters.
I have used both kinds in the past and have noticed no difference between them in normal operation.
The existing lamp holders in the Turret fixture are modified for the F90T17s so you don't need to locate any of the original type of holders.
Unfortunately you are correct about the F90T17 bulbs as they have very difficult to find as of late.
I figure that they will be showing up on ebay at some point in time, but so far no luck.
Also If you can't find any F90T17s, you can still make the same fixture using F40T17s which are still available on Amazon and use a Advance SM-2E40-S-TP instant start ballast or similar to operate the F40T17s.
The fixture would look just the same, but would not as much light output as the F90T17s