Threesome

So we and my partner have got to that stage where we want to try new things. We were considering a threesome obviously there's talk on what gender (probably girl) and age etc. I've said I'm not comfortable with the other girl having sex with him at the moment which he's fine with. He said he won't do anything I'm not comfortable with. 
But where and how do you sort these things out? We want it to be someone we don't know and will never see again... But not sure how/where we get someone 😂😭