Haberdashers/Fabric/Materials shops have been the best for me, especially if you don't mind it being blue and are handy with an electric carving knife or hot wire cutter.
Other than that you're looking for "pick and pluck" foam, so its whatever size sheets you need.
http://www.google.co.uk/search?q=cu....,cf.osb&fp=105b164de5fcaabb&biw=1280&bih=844
http://madmodder.net/index.php?topic=4770.0 - if you want to get a little bit more custom.