Yep I am a teabag fan, Just take any teabag snip off the end with the string attached. Unfold it and empty the tea. Now you should have a tube of empty teabag (4"x2" aprox). cut about 1/2"-3/4" (depending on the type of cartridge) off of the end so you should have a piece about 1/2" x 2" roll it loosely and stick it in the cartridge and load 'er up.
But alas I have moved on from teabags also. NOW using felt square from craft store. Any craft store has what are called felt squares. About the size of a normal 8 1/2"x11" piece of paper. Price varies from $.20 to $.50 such a deal. I cut a 1/2" by length of cart, fold it once and stick that puppy in. Works like a champ!! and not a lot of fussing with - just need to determine the size that works for your cart. one fold in half is certainly the right thickness.