I used to order "Black Knight" bettas from a fairly local supplier in Minnesota (now closed after the owner's death) that were easily over 3" every time. I think 2 of them were 4" and I wish I had bought one before I knew how hard to find they were. :\
I tried googling them but I couldn't find one quite like them. The black one on this page http://flowerhornrusforum.com/topic/106765/1/ looks similar but the ones I was getting were a longer shape (very thick bodied though), their tails were perfectly round, seemed a little small for the body size and were set off a little more so that they didn't meet with any of the other fins. They weren't cheap but every one came in huge and pure, velvety jet black.
If I saw one today I'd snap it right up.