Take it from someone in the field: websites are not always a good indicator of what is actually available. If you keep an eye on the weekly sales flyer/newspaper ads you'll probably find it listed there sooner or later. (Story time: One art store stopped listing a specific brand of face paint on their website and I panicked thinking they were not going to carry it anymore. They still carry it everywhere, but it took them a few months to get the website up to date. They were adding more colors and stuff, but for some reason the listing was taken down for a long time during that update.)
They could be updating the website, or it could be a clerical error when they last updated stock, or that line could be listed under a new name. Or they may have just stopped offering to sell it online for the time being but it should still available in stores. The Casa Collection seems to be a staple product for Joann's, and they always have huge sales on it come prom season so I can't imagine stores would suddenly stop carrying it.