It's tough to help you out because only a "handful" (pun intended) of current MPAs offer this. Also, prostate massages can range from mind-blowing to painful. It really depends on whether the MPA actually understands where the prostate is, the right amount of pressure to apply, the timing of that part of the massage, and has a playful (rather than clinical) attitude about it.
I have had excellent prostate massages by MPAs, all of which have left the scene. I've also had a number of probing exercises which were at best forgettable and at worst painful!
One of the side effects you should expect from an excellent prostate massage is that your juice may fire quite a bit more forcefully than normal, so turn your head!
My advice is to find someone who advertises that they have been formally trained in Chinese massage.