Endometriosis Support Group
Endometriosis is a common medical condition where the tissue lining the uterus is found outside of the uterus, typically affecting other organs in the pelvis. The condition can lead to serious health problems, primarily pain and infertility. A major symptom of endometriosis is pain, mostly in the lower abdomen, lower back, and pelvic area.

Not all REs are equal though, so you'd have to check them out.
I was told to stay clear of oncologists unless I have cancer. Their focus is saving lives and less on preserving the reproductive organs.
I'm currently seeing an OBGYN, but I'm not having excision surgery with her. She's doing my yearlies and monitoring my chocolate cysts. If I go with surgery I'd definitely seek out an RE.
It the doctor is good though, and you like them that is all that matters. I had to visit a couple of gyns before I found one I really liked and felt comfortable with.
Personally I looked up a lot of reviews and recommendations on endo sites to find the one who did my excision and I am so glad I did.
My ob/gyn, who I found through the gynecology service website of a hospital located near me which happens to be one of the best hospitals in the country, is a RE. A lot of what she does is infertility, though she also has a lot of experience with pelvic pain and si trained in laparoscopic surgery (which not all gyns are). If you're looking for someone in a subspecialty, RE will be the one, but many great endo doctors out there are board certified as general ob/gyns not subspecialists.
I've also heard talk that endo/pelvic pain should be added as its own subspecialty. I wish that would happen, would get the acadmic medical community to talk about endo a lot more if nothing else, but don't know how likely it is.
Anyway, checking out the websites of good major hospitals near you and seeing if they have a "find a doctor" search function is a good way to start.