The following links are to resellers I've had really good experiences with. All of them are first rate and are easy to deal with if any problems should arise.
Anime Chaos
Over half my cels have come from Anime Chaos. He usually has a great selection at decent prices. Updates are frequent too. Nick's also a great guy to deal with.Animanga
Yann's site has ALOT of cels to choose from. Updates are frequent and prices are reasonable. He's in Switzerland, so expect to pay more for shipping. He does take credit cards though.Splashpage Comics
Sean's site has alot of excellent quality cels, but they're on the expensive side. Still, you get what you pay for. I've snagged some of my best cels from here. They also take credit cards.Asylum Anime
Asylum also has a good selection of harder to find cels. They also update fairly frequently. Credit cards accepted (notice a trend here :-).Robert's Anime Corner
Bob only updates quarterly, but the cels that come in are always top notch. He's also a super nice guy to deal with. Customer service is his number one concern. He also takes credit cards.