The best place- A vet that treats BPs, or a breeder.

From there, you will just have to get to know people you run into at the pet stores or word of mouth, then ask them who can do it.
Use judicious sense though.

There are two ways to sex the snakes: popping and probing.

Probing with the proper tools is the most accurate, reliable way.

Popping is somewhat controversial, unless you are experienced and know what you are doing, fine. The problem is, someone with no experience may have seen it done on you tube and now wants to practice on your snake and can harm him.

Now probing can be easily learned on YouTube and the tools bought cheaply enough on Amazon.

There may even be a "Sticky" thread on probing here, or do a search.