I had one with two guys once, it was completely spontaneous and I wasn't seeing either of them, though one of them was my ex. It happened in the back of a car (!!) and it was pretty good. If I had that time again, of course I'd do things differently, but now I don't think I'd have one again. Everyone is different and I personally wouldn't like the idea of a threesome with my bloke because I'd be worried about damaging the relationship.
If you're confident this isn't a risk then go for it. Just make sure you've discussed all the implications and that you practice safe sex. Online is probably the easiest way to find someone, alternatively look in Desire Magazine for someone, or advertise yourselves.