This can be a little harder to find then just a couple for swapping, however you can find a lot of people out there looking for a lot of things. Additionally there are a lot of couples in the lifestyle who love to adapt to the desires of other people and see where it goes. I suggest you hook up with Adult Friend Finder and probably Swing Life Style and set up a profile. Carefully describe yourselves and what you are looking for. The only thing you should be warned about is that there are a lot of bi guys out there who will probably reply to your profile. Just sift through them if you are interested or avoid them if you aren't.
Be clear about what you want and you will probably find people interested in it. Bisexuality is more common for women, but there are some men out there who are either bi or curious...