You might have a look around at www.gemfix.com Very nice yellows there, and excellent (too good?) photography - great to get an idea of the general color you want.
Ditto the above.
Budget - so we'll know how easy/difficult the search will be
Shape - ovals are the easiest to find. Cushions are okay too. Pears, step cuts and rounds are not as common.
Colour - a few example pictures will be helpful.
Treatment - heated is easier to find and more budget friendly. Unheated will be more challenging.