From past experience I had trouble finding the holders as well, I ended up going to Ikea and buying some cheap plastic CD Holders and cutting them up, reason was because I wanted something with a 'spring-like' mechanism. But you can always just make it out of wood yourself by cutting the slots with a router or table saw.
my school orderd a big lot of them from gah one of the companies my dad uses cause i gave them the details... but I forget what it's called now.....
They make laminate, laminex maybe? nah i dunno, just look up the companies that make hinges and doors ect..