Chard56 has some excellent bettas.. he is on the forums here as well.
But as mentioned above, it may be harder to find ones to be shipped.. you will also need to time yours right so that you will be able to ship when it is warmer as well.
I personally think you can't go wrong with marbles- whites, yellows, coppers, oranges, true greens and blacks seem to go quicker.. also the most purple looking go quickly as well. Dragons you tend to not be able to go wrong with, always have a demand for them.
PK, HM, D(bl)T, SDT are also very popular.. Breeding CT x HM has also given rise to the development of what is commonly called a HALF SUN -- Combtail Halfmoon. This is a very beautiful type. Too bad heart and triple tails aren't so common now :(