If you go the doctor route there are a couple notable ones - one Dr. Cahill in NY but he is hit and miss and under doses medication but his exam can find the cause for some people. He didn't help my diagnose tho but some have got some results with his test.
There is also Dr. Yu in St Louis who prescribes parasite medication and does alternative testing. While he didn't help me totally (I'm sti deal with stuff at this time ) he does seem to be more aware of the parasite issue than most doctors. His site talks about his practice - preventionandhealing.com
The standard tests seem to rarely pick this stuff up tho so it can save money to not waste time with the 'regular' doctors if they aren't helping. Even parasitologists can be a waste of time.
I traveled over 10 hours by car to a parasitology center in Quebec and the doctor there was a stuck up ***** he gave one measly blood test and a stool test and that was it.
Another lab that can be found onine parawellness.com has a potentially good stool test. The doctor won't prescribe meds but may be able to actually examine the test if anything shows up. Most labs they barely examine the samples and probably just put it in a machine to check for a couple things but this doctor actually takes his time to see if something is there.