I've never seen it at any of the chain stores... If you go to plaster.com and click on the "Find a Local Distributor" link at the bottom left of the page, you be able to search your state for local distributors and which types they carry. You can also order it from that site as well. A lot of ceramic/pottery suppliers carry it as well as building and masonery suppliers.
Yup! Mannowar is right. Check out plaster.com, and search through their list of local suppliers. However, I've found that it's very innaccurate. Out of all the Kansas City distributors they listed, none of them carried UltraCal 30. Furthermore, none of them had even heard of the product. I ended up just ordering it from Plaster.com.
If you do order it, be sure to get yourself an N95 approved dust mask before you start to handle the stuff. It contains Alkaline... which is an ingredient in batteries. Yummy.