Well, you didn't say if you wanted carpet in the cab, or carpet in the bed, or both, but...
If you're looking for cheap bed carpet, you have two options. You can go to a regular carpet store and buy a remnant that is big enough for you to use. You don't have to go full, bulky 70s shag, as there are some pretty cool, sisal-type carpets out there that are relatively cheap.
If you're concerned about water (running with your top off in the rain, or muddy dogs in the back, for example) you can find a marine supplier in your area and buy some marine (boat) carpet. Not as cheap as home carpet remnants, but still pretty easy on the wallet.
Carpet in the cab is a different story, as most carpet kits are molded to fit the floor pan. I'm sure someone out there has used regular carpeting to carpet the cab of a truck or Blazer, but I haven't seen any, and I'm not sure how hard (or easy) it would be to do.