So you know what your looking for.
http://www.ferry-morse.com/seed_starting.asp?folder=productssplash&type=peat% 20pots
Reminder that peat pellets expand when exposes to water and contain a potting mix that has trace ammonia.
Remember these are compressed peat. So use dilligence when adding to an established tank.
Thats an example and you can find similar products at Ace, Home Depot (Go 20), Lowes, and probably any gardening center.