Catherine,
The only place I know of that still sells it is OA. I just used it the other day on my tank and as long as you've got Carbon going, it should go okay. If you're treating your display and you think there are quite a few flatworms...be ready to siphon them out as they die rather than counting on the carbon to do the work for you.
Here is a good link to look at:
http://www.melevsreef.com/flatworms.html