Hi @Cball
It would probably help members here to assist you if you clarified whether you are seeking to hire paid companions, or are looking for non-commercial encounters.
If you are interested in hiring:
Several years ago there there was someone with a similar interest in Muay Thai boxers who posted on the boards (I know @bucknaway also has this interest, but I think this encounter involved someone else). He frequented a girly bar complex in Pattaya (between Beach Road and the Pattaya Beer Garden) where they had set up a boxing ring, and retired Muay Thai boxers gave exhibition matches and then mingled for tips. He got the boxers attention by giving them generous tips and buying them drinks. After a night or two of this, he befriended one guy in particular and made an “indecent proposal”. If I recall correctly, the boxer took him up on it, but being straight I think he was unwilling to do much - it was basically a muscle worship session. Obviously this modus operandi was perilous as the boxer might have take the proposal it badly. In any event, I think that bar complex is now closed.
Currently, the “Pink Panther” bar on Patpong Lane 2 in Bangkok also features Thai boxing exhibitions. It’s a straight bar, though, so the boxers may not be open to such proposals—caution is advised.
You might also check massage parlours whose websites list masseurs’ interests—some mention boxing or other sports. However, the guys that mention boxing seem to be the more the straight guys from memory, so I’m not sure how much they would be into "sweaty rough sex with sweaty rough dudes".
If you are into non-commercial contacts:
Try scrolling through Grindr in Bangkok and filtering by interests—some users may list boxing or similar activities.