Screwtop bottles can work with a bench capper, and I have seen some people claim that they have been able to get a seal with some wing cappers.
The biggest problem with the screwtop bottles is they are weaker at the top around the threads, and it is easier to break them when capping.
Check CL for bottles, I recently switched to kegging and kept only 100 of the 400+ bottles I had aquired, I sold about 325 bottles to a guy for $25, he actually gave me $30 because I threw in a couple of milk-crates. Either way, I see bottles for sale on CL here in my town all the time.
Also, if you have a neighborhood recycling program, take a stroll and scavenge from neighbors recycle bins, or if you have a community recycling bin, jump in there and see if you can find bottles that way. I have no shame, I've done it