Most clubs prefer their tug pilots to be glider pilots too, although it is not a pre-requisite. If you wanted to do a season as a summer tug pilot, then a few hours under your belt on type would help, bearing in mind that most (but not all) tugs are tail draggers.
If you want to do a bit in your spare time, the best bet is to find the most conveniant club that does a lot of towing and join. Once you've got your foot in the door, and done a bit of gliding, it shouldn't be too difficult to get in the tug. This may seem a little expensive and long winded, but compared to hiring an aircraft, it only adds up to a few hours, and could be far cheaper in the long run.
Besides which, gliding is great fun, and you're bound to make a few contacts along the way!!