Some gliding clubs will supply Dynafoam and charge by the square whatever - suggest you ring round and ask. I simply asked at mine, but it's rather too far from where you live! You can't expect them to let you have any of the cushions kept at the launch point (or wherever), they are for the club gliders.
The alternative is to use Google and see who sells it. I imagine a sheet will be a lot cheaper than getting it cut exactly to size. It's easy to cut with a Stanley knife or similar.