When I had that problem with my TR3A, I bought some "cut to fit" shims at my FLAPS that worked well. They were just soft sheets of aluminum with adhesive backing, that could be cut to size with scissors and glued on the back of the pads.
Another trick that sometimes works is to use a file to bevel the leading edge of the pads a bit.
If all else fails, new pads may be the only cure. When you do, be sure to scuff up the rotor surface and clean it thoroughly.
BTW, TRF shows the shims on sale at the moment, which probably means they are in stock