I'm not sure anon cump-dump sessions are really a thing in Thailand. Christian's suggestion is pretty good but in an O-party you can't be really that anon and guys would probably want your explicit consent before doing anything.
Sniffies.com which is pretty popular in the States for this kind of activities (not sure about the UK) has pretty much no users in Thailand. I opened it last year while in BKK to show it to one of my Thai friends and only saw one profile in there. Doubt the number of users has gone up.

I guess the only solution I would suggest is to pay some local guy to recruit other guys and send it in your direction. I'm not that familiar with transactional sex in Thailand, but definitely it won't be that cheap to set it up.