I buy a lot of stuff like that from Texas Art Supply ( www.texasart.com ), but that's because I get to go to the store and look around. They do mail order, and they generally have pretty good prices, though I'm not sure about shipping.
I've used Celluclay before, and if I remember correctly, it worked pretty well. More importantly for me, I found it to be very easy to use, to the extent that it made up for other shortcomings of the product. And you get a fair amount of it pretty cheap.
I hope you make it through the storm okay -- today is the first day we haven't had any real rain in about ten days, so I feel for you.