The best places to find another girl for a threesome is the strip clubs or swinging clubs. Or you can do it the easy way instead by renting out an escort for a couple of hours.
I have had 2 threesomes in my life, but it wasn't with any g/f. I rather prefer one on one instead. I personally would never have a threesome with serious g/f because I have a feeling that it can have high potential of ruining the relationship. I have seen it happen before, but that's just me.