Maskol, its all the rage, and I am raged

I would think that any source of Humbrol paints would stock it. On line try Roll Models or Squadron. There are also several different manufacturers that produce their own version of it - Micro Scale comes to mind. I even have a bottle of Maskol type fluid that I purchased in a craft shop while visiting with my wife - it's an off the wall brand (ie I've never heard of the manufacturer) but it works just fine.