Like Kermit said, Salvia leucantha does set some seed but is very stingy about it. I've never seen them available commercially.
I got some seeds once in a trade from someone in coastal California where the plants bloom almost all year round. I also collected a few seeds from a 'Cislano' plant that I brought inside before the first freeze one year. (Sadly, 'Cislano' is no longer with me.)
