You can find recoil pads on ebay, amazon, or one of the online retailers. You can also try your local gunsmith and see if they have any used ones. Get one that is a little big and then file it to fit. You can also make a spacer out of wood or plastic and use the existing butt cap with some longer screws.
The simplest way is a slip-on butt pad, which will run $20 - $35. Almost as easy but way cheaper is to get longer screws and spacers. Some hardware stores sell cylindrical aluminum spacers. Tension pins will work also. They're a little hard for a hacksaw, but a Dremel will slice through them. I suppose you could even use a pen barrel cut to length. If you want a little thicker look, slide on some black vinyl tubing. If you don't like the exposed wood end grain or the underside of the pad, make plates to cover them up. (Same goes for the old screw holes if you move the pad up or down.) You can cut plastic from oil or antifreeze containers (check your county recycling center). If the material is flimsy fender washers will hold it flat against the wood or rubber pad. Or make the plates out of metal - Walmart has a stovetop pancake griddle for $13 that was enough for several of my guns. Or come up with adjustable contraptions with sliding parts. Or buy a thicker butt pad and install it in the traditional way. There are lots of options.I have pretty much stopped making things adjustable. I found I was putting a lot of effort into the design, only to never touch the adjustment again (because it was sized for me). Better to make it fit me in the first place.