I try to use hardware store or commonly available products as much as I can, partly because I'm cheap and stubborn, and partly because I live in a small town. There's lots of good stuff out there!
I do use hardware store plumbers epoxy putty a lot, but make sure to test whatever you buy before putting it on your model! There are a lot of varieties sold at hardware stores, and they're not all suitable for modeling. Some cure sort of rubbery, others cure VERY hard and are impossible to work after curing without power tools. The kind I use is dark blue with white center. I've tried various brands, and the ones with this coloring all seem to be the same. I actually like it better than "modeling" epoxies for most applications. I have tried some that was grey (cures slow and hard), light blue (rubbery), copper colored (rediculously hard), and silver (hard). The dark blue is the only one I can use for modeling. I'm not sure if the same will be true for what you find wherever you are.
Another great filler (hard to know what you need without seeing a pic) is superglue mixed with corn starch. Just mix it on a piece of wax paper or an old yogurt container lid, in approximately equal parts (the ratio depends a little on what you're doing and the viscosity of the glue you use). It cures up pretty hard and fast, but sands and carves well. If you're trying to back-fill a part that was eaten by the Tamiya putty, this might be a good choice. Once again, play with some tests before you put it on your important project.
Or there's polyester putties, 2 part autobody fillers at a hardware or auto parts store (Bondo here in the states). This stuff cures a little softer than the above options.
If you post a pic of the damage I can be more specific about suggested fixes...