I want a LPS system, but I'd rather not pay $300 for a ballast. I've heard of people using fluorescent ballasts to run LPS bulbs. How does that work and what do I need? It'd be nice to have at least 90W, but if that's not possible I can deal. Also, are there any other sources for LPS bulbs besides 1000bulbs?

With the ballast you have to look for operating current (it has to match, LPS is not as tolerant as fluorescent: It withstand no overcurrent at all and on undercurrent the efficacy drop and warm-up time rise significantly, so no more then 10%) and similar arc voltage.
In order to get them you have two options:
1) Find lamp/ballast combination meeting the above (so you are very limited in bulb wattage selection; look for instant start ballasts)
2) Design your own ballast for the lamp you want...

Eh... Google tells me LPS requires 300-700V depending on wattage. Fluorescents need more like 70-100V don't they? It doesn't look like any match up at all.

I've had success running LPS lamps on a electronic instant start ballast for T8s as most of them have a OCV of around 500-600V which should strike a LPS lamp. As for which kind of IS ballast it depends mostly on the wattage of the lamp you're planning to run.

I didn't buy anything from these stores but both prolighting and lightbulbdepot both sell LPS lamps and ballasts and I think the latter store also sells the BY22D socket for LPS lamps but all this stuff is really expensive.   

So would something like this 96W ballast work for a 90W LPS bulb, or this 32W ballast for a 35W LPS bulb?

Neither of combinations would work.
SOX18 require 0.35A,
SOX35 and 55W require 0.6A,
above 0.9..0.95A.
wfile
F96T8 require ~0.5A
F32T8 about 0.36A

The closest combinations i found among US lamps are:
F96T12 (0.85A) with single SOX90W
F32T8 (0.36A) with double SOX18W in series