I've been using Holcroft Flow Medium this year, and I'm loving it. It's not gaming specific, you can get it (or other brand) in any good art store. I think from memory it cost me around $20 AUD for a 250ml bottle.
I bought some small needle nose dropper bottles and put some flow medium in those. That way you can measure out a more precise amount. And yes it works well with model paints. I mainly use Vallejo paints, but also have a number of Citadel and even older Humbrol paints. Seems to work with all those paints ok. Been using about two drops or so with my paint, and that seems to be the right amount most of the time. But like Macross said, every paint/colour/mimiature and even day/humidity can play havoc with thinning paints at times. Add more if you're more aiming for a wash/glaze.
I've also put in some kicker for ca glues in the little dropper bottles as well. This works a treat, instead of spraying your entire figure, hand, and your table with kicker, you can apply the kicker exactly where you need it. Only thing is that these bottles don't seem to be able to keep the kicker from evaporating, so only put a little in when you need to use it. Otherwise it's a waste of kicker, and that could be expensive.