yea, try looking in the fish tanks, that's where the stores around me keep them. but a lot of time, unless they just got them in, they'll look gray from the grit and nastyness, i'd avoid those. a little could probably be washed off, but don't waste your money unless it looks mainly green.
when i had a marimo i never did anything for it, and it lived happily for a good month or two, until it got poisoned by toxins in the water and had to be thrown away. they're super easy, and don't need anything other than a little light and to be turned every now and then so the bottom doesn't get brown.