Lukas, the cel curator is very helpful as well as knowledgable. Animanga Based in Switzerland, Yan provides a wide selection and good prices. Haven't bought a cel from Yan, but have asked him a few questions once in a while. Anime Assault Cesar is great at getting back to his customers that has any sort of inquiry. He also has a good selection and is growing quite rapidly.
Prices are very good.
This is Chad's page. This is the ultimate guide to locate cel dealers and collectors. S Mak's Cel Gallery Stephen's marketplace is small. You never know what he'll put up. An avid collector indeed.