i-mockery.com has a nice little roms section, not all of them have a downloadable ROM, but most do...check out the rest of the site too, lots of amusing little thing hidden throughout. Pixel Pals is my favorite section, 4 panel comics using NES screenshots.
Lets cut right to the chase and be done with this:
1) First, check USENET
2) If you aren't patient enough to see what cycles through Usenet, just go with one of the Rom Burners who do the MAME rom updates on DVDR. For Like $10 or $20 bucks you get a DVDR with every rom, plus variations, period.
I downloaded every nes rom in one download from supernova before it shut down. All these different rom sites out there are a huge waste of time. I'm sure there are still some other bittorrent sites out there that you can find them on.