I recently saw an old possibly abandoned barn in my area with a Line Materials Unistyle 175 still attached to it, and as far as I know doesn't work and is disconnected from power. I want to possibly get this light as I don't have any streetlights yet.

Does anybody have any advice for finding out who owns the barn, and how to actually get the thing?

It's a long shot, but it might just work out.

You could try doing an address search on the local property appraiser’s records to see who the owner is.

There’s usually a mailing address on there so you could try writing them a letter, explaining that you’re interested and sling if they’d sell it.

Not all places have online records though.

But my guess you won’t get a reply. But you never know.

Another method I heard some members doing is to leave a note explaining you're a collector and interested in the light with your name and contact info and hope you get contacted by the owner.

 You can also talk to the property owners around the barn I'm sure one would know who owns it .

 As for getting it down . How long of an arm is the light on ?  If you can easily reach the light itself , remove the door/refractor ass'y 1st ! The hinge has a spring loaded sliding pin which from inside the fixture releases the door from the main body . Then you can un hook the feed wiring , loosen the slip fitter bolts and slid the light from the arm . If your keeping the arm , that's the easy part as if it falls , it's not gonna hurt it .

It is kind of high up. Here is what it actually looks like.

 That doesn't look too bad . You'd definitely need a decent extension ladder. Plus, with it being on a really short bracket will make removal less difficult . You'll definitely need some help though .

My dad does have quite a long ladder. I think it is around 8ft folded over and even more when it is just straight. Do you think that would be long enough?

 Possibly !

There is a house across the street from it, so the homeowner there probably owns it.
Now the hard part is to get my dad to drive me there, and actually ask the person if it is their barn, and if they would let me have the light.

If you're lucky you might score it for free but if not I suppose you can offer some cash or to trade the fixture with something like a LED.

The barn is abandoned, and I don't think the light has been connected to power for several decades.

 Good Luck !