You can buy high density foam online or at some large fabric stores. But it is not cheap. I paid $25 for a piece that was 2" thick and 2' wide and 4' long....and that was during a 50% off sale.
I used it to modify and customize our sex swing and we are thrilled with the results.

The foam was easy to cut to size using an electric knife. You may encounter more difficulty working with a thicker piece and cutting at an angle. You'll also want to make an appropriate cover, so in the end you may not save that much money.

Good luck!

Yeah, the prices they charge for them are highway robbery (not your fault, EF!) You can easily search for wedge pillow or wedge cushion and get one at at any online department store for like $20-40. Or you can use my method and just use some of the more firmer pillows you have lying around that way you can vary the height to whatever you need.

Nope but it's got to be easy

Wouldn't a couple nice pillows do the same sort of thing?

You can get a wedge pillow for medical purposes and re-purpose. They make wedge pillows for acid reflux and leg issues. I found some for around $30 inc shipping. Just search for wedge pillow or bed wedge on one of those big sites. I don't want to list name of site b/c not sure if against EF policy. Message me if you need the name. Looked into buying the foam but it was more $ than buying one of these pillows.