We went out to BDSM munches and socialised. I found him a couple of girls that way once by striking up conversations with other female subs and warming them up for him. Be aware that even in the supposedly famously open-minded kink community you're unlikely to find anyone who actually gets cuckqueaning, and there is a chance that creepy dudes will treat "I already have a boyfriend and Master" as more of a suggestion/challenge. Ugh. Corona-chan has killed munches dead in most places but they might start up one day.
If you aren't good at it but still want to feel involved or like you're cheering him on, then try training yourself to be aware of other girls, check them out on his behalf, and point out to him ones that you think he'd like. Two sets of eyes are better than one. This assumes you can go out with him, which in some places still isn't possible.
One time when we were still experimenting with the idea I paid for my man to see a higher-end escort and helped him research it. It doesn't give you the full "punch" but it was okay as training wheels. Only applies if you live in a place where the sex trade doesn't consist entirely of those who are trafficked, STI-ridden, worn-out, or all three.
Never tried dating sites. He had some success with Tindr, so if you're okay with an Israeli company capturing rich analytics on you/your bf's attraction analytics and preferentially trying to match him with races different to him, I guess you could try. You can help figure out what photos of him to use or take new ones. You could even swipe on his behalf if you want to be more directly involved. We found that being upfront about his girlfriend wanting him to fuck other girls had a strongly negative effect on Tindr matches.
I advise against thirsty friends unless you're one hundred thousand percent sure it won't get weird and/or leak out wrong into your social circles (not fair for him to get a rep as a cheater because you told him to cheat), and I advise against exes for the same reason. I hear hatefucking exes is a thing though so who knows, maybe it'll work. If you do contact one of his exes on his behalf and ask them to fuck him then absolutely come back here and tell us how it went, I could use a chuckle.