If you sew at all, it is not that hard to make saddle pads. Make a pattern from a pad that fits correctly, use quilt batting for the middle layer (more layers for a thicker pad, fewer for a thinner one), and a heavy cotton like denim or twill or a heavy duck cloth for the top layer. I use towels for the layer against the horse as it wears well, absorbs sweat, and washes well, but you could use the heavy cotton top & bottom if you were planning to use something else between the horse and the pad. If you do use towels (terry cloth) for the bottom layer, buy big towels instead of buying terry cloth by the yard, as the terry cloth is good towels is of a much higher quality than the terry cloth you can buy at a fabric store.
There are also places that will custom make saddle pads.