Tufa is very nice for tanks where you need buffering, like marine aquaria and Rift Lake Cichlid tanks.
The big issue with ordering some in the mail is that, since it's rocks, the shipping charges will be quite high.
Check at places in your area that sell rocks for landscaping and building supplies. Most towns have stone yards specializing in rocks, gravel, sand, and so forth. Many will sell either by the pound or by the truckload. They are less busy this time of year, so they might offer you a good deal.