Where is the best place to buy diamonds?

I need to purchase a diamond but I have some very specific parameters, where is the best place to buy a diamond that has a great return policy in case it’s wrong in some way and that also has so many diamonds in stock I can filter ruthlessly and still have a few to choose from? I tried James Allen but their minimum size is too small. I ideally need one between 0.2 and 0.3 caret (I have the ring in mind I just want to replace an emerald for a diamond, and the emerald is roughly 0.2 - 0.3 caret). Thanks