Obviously, the best way is to get them directly from the manufacturer. But even if you won't be buying the $6,000 worth in 6 months, I think it's still best to buy from them to begin with.
Your initial order only has to be $300. So you will spend much less per shirt than if you got them from anywhere else.
I didn't spend the $6,000 in the first 6 months, but I bought decent quantities from them on a regular basis and they were able to see that I was serious about buying from them, so it wasn't a problem.
If you are just looking for a wholesaler of ProClubs,
PRO CLUB T-SHIRTS is a good place to start. But, again you will be paying much more per shirt, and they still only sell per size, per dozen.