Are NES cartridge EPROMs erasable, or do I need to use purchased ones. I have a UV Eraser coming in the mail.
NES cartridges don't use EPROMS unless it's a prototype or development cartridge. But if they do use EPROMS, then yeah, they're EPROMS.
The vast majority of NES Game Paks use mask ROMs, which can't be rewritten any more than the CPU's instruction decoder can be rewritten. You'll have to do some soldering and probably some rewiring if you want to use UV EPROM chips.
If you want a simple cart to make your own games with EPROMS, I built a dev cart for NROM-128 games (that's 8kb CHR and 16kb PRG) tthat utilizes two ZIF socket so you can just plug the games in. Works great. However, I don't need it anymore since my own project has upgraded to use U*ROM carts. It also features a switch on the outside of the cart to select mirroring. Write me if you're interested, I'm asking $35 plus shipping and handling (it was quite a project, at least for me). It's pretty slick. I can provide pics on request.
Also, EPROMS don't need any rewiring, at least not on NROM carts. Hence how I was able to simply socket them.
I got my EPROMS from Futurlec. Be aware they ship out of Bankok Thailand, so they take a long time, but the prices are good.