I grow this plant and I adore it. It's huge and blooms forever. I purchased mine years ago from a local Canadian Tire that must have had an avid gardener in charge of ordering their plant stock because I would often find gems there. I have not seen it offered in any garden centers since then. I did though do a quick look and came across two online sources in the US. One is Odyssey Perennials, the other Eminence Meadows. Good luck.

May I suggest the online nursery geraniaceae which specializes in geraniums and pelargoniums.

Not only do they sell psilostemon, they also sell 2 cultivars of it. They are an excellent nursery that I highly recommend.

Here is a link:

geraniaceae