Humbrol maskol is what you want. Just do yourself a favor and grab some off ebay. Where local hobby shops are getting harder and harder to find, and they don't all carry a full line of humbrol products. Your best bet would be to make one large online purchase of all the colors you'll need.
Michaels, and AC Moore, even Joanne fabrics, will carry winsor and newton masking fluid. But it dries way too clear and is really hard to see what you've already done. It will work in a pinch, but not ideal by any means.