Not only at garden centers! It grows away quite happily all over the place without any need for human intervention. It is a very common grass, just go to your local golf course. Look at the grass that grows around the ponds. Chances are there will be large quantities of Phalaris Arundinecia.
There are however, a few significant drawbacks to Phalaris grass as a source for spice. It isnt particularly potent in comparison with Mimosa, and the concentration is highly variable from zero to not very much. It contains other alkhaloids far less desirable (toxic) which are unavoidably extracted with the spice. Extraction requires more effort with extra defatting stages thrown in.
There has been limited talk about Phalaris Brachystacys on this and other related sites, with promises of higher yields and kinder alkhaloid concentrations. But while Mimosa remaians legal and easy to aquire, cheap and simple to extract, with out any of the nasties, I doubt there will be much interest in any Phalaris type grass.