If all else fails, try Hobby Lobby (Crafts, etc. is online supplier). I can't find Celluclay on Crafts, etc, but Hobby Lobby is ususally a little cheaper than Michael's
http://www.hobbylobby.com/catalog/catalog.cfm
If you look at the SPECIALS page, then go to Regular format ad, you'll find weekly sales. Believe me, I look every Sunday for things like Celluclay, because 12 bucks beats 24 bucks even in Trump's limo. If it goes on sale for 1/2 off, I could send you some, as they also sell 1 pound blocks. Either way, with shipping, it's bound to save you a few dollars. I owe you the favor.
Sculptamold is a similar, cheaper version. Have you used it? I've used both. It's a little more coarse, but lighter. Also available at the Lobby.
I love the Hobby Lobby and I want to live there. Why won't my wife let me?
Steve