These people organise them once in a while:
External Link/Members OnlyAlthough they don't look great to me, tends to be one or two overweight women lol. They have done better ones in the past with fit girls but not for a while so worth being aware of and watching
Or you could go to a swingers club or sex sauna type venue, not quite a gangbang but basically the same vibe if you go to a good one at the right time
Otherwise, just find a few people on here that would wanna be involved, hire a few WGs and make it happen. We've done that here before and I was lucky enough to attend one. Although, not as easy as it sounds as you have to find trustworthy people that won't drop out, get them to chip in, find enough girls that are free at the same time, ask one of them to host or if not find a good venue, etc. Easiest way is probably to find a girl that's both done it a few times before and is willing to host, as she may be able to help organise it for the right price. Most girls will want a deposit to arrange it which involves getting the attendants to pay a little up front, unless you wanna cover it yourself but I assume not.
This seemed to be something Welsh Brunette was into for a while but haven't heard about that coming up in a while
If you're looking to not pay for it and are willing to put some leg work in, the other option is to join Fabswingers and get a few couples involved. But obviously that's quite a bit harder than any of the above