As Scott said, there's a lot of ways to do it yourself. Using putty or Miliput with a screwdriver is another option.
Or there are ready-made solutions from Eduard, Cavalier or Atak. I'd recommend the latter two as they are very thin resin sheets - easier to work with & more natural looking than the Eduard brass.
I wouldn't recommend the Tamiya stuff. It lacks depth & definition.
This is my favourite video tutorial on this. Ok, it's a panther but I like the quick simple way he does it. The Tamiya tools are cheap and readily available online. I also like how thin he gets it, far too often you see tigers and such looking like they have been iced like a cake :eek:
A very good example of self made zimmerit! If anyone tries this, roughing up the plastic surface helps the filler grip & leaving it on the model for a few minutes before using the tools is useful too.