Thanks for the suggestions, Jan. It's not available at chemicalstore.com, but it is available at chemistrystore.com - but 1 pound is the minimum, and there are some issues regarding hazmat shipping. Most pet hobbyists would find this a deterrent.
I did find it on eBay, being sold in smaller amounts for treating koi. So perhaps some good pond stores and pond supply websites may also sell it.
I don't know why petshops don't sell some kind of product for this purpose! Many aquarists have learned the hard way that live plants are a good way to infest your tank with snails, etc. I tend to see these things as beneficial, rather than pests, but other people are more fastidious than I am.