Wondering if my 250w PSMH ballast can run anything else, specifically any HPS. The bulbs are kind of hard to find.

maybe a 400W MV (a bit underdriven), but with the ignitor disabled.
HPS do not seem to be that far away from the nominal spec perspective, but HPS are very sensitive specie, not only to the exact current, but mainly on how the current changes with temperature. You may try it, it will either run underpowered (check the lamp arc voltage at that state - if signifivcantly below to what the MH uses to work at, it may damage the ballast), or cycle if overpowered. Check also the lamp power (measure the V, A, the power is then approximately 0.9*V*A), if it isn't above the rated 250W, as that would wear the HPS quicker.
 

Notwithstanding Medved's reply, I would always urge caution. By and large, it's always preferable to run a lamp on the ballast it was designed for, and the same holds true the other way around. A 250W MH ballast was designed to run a 250W MH lamp, it's that simple, for the most part. Here in the UK, HPS and MH lamps are usually (but not always) electrically similar, so will run on the same ballasts, but that doesn't hold true in the US, as I understand it.

By all means experiment, that's how you find things out, but take care.

Here's a lamp for $10 O.B.O. + shipping  :  https://www.ebay.com/itm/166480430347   THere's also plenty of others for decent prices on eBay .

The best and most versatile lamp for this ballast would be a Philips Allstart CDM205/U/O/4K/EA...best performance too.