Look at the oled-companies index - there are many OLED makers and resellers - Samsung, LG, OSD, etc.
Well, I think the only companies making 3" panels and selling them are Samsung SDI and CMEL.
Smaller panels are sold by many companies, I have been in touch with OSD and had good experience.
Still looking? Try www.Densitron.com
They stock samples, evaluation kits and know a lot about driving these displays