If you can find it locally, then you would be a lot better off. Start with Home Depot, Wal-Mart, and Lowe's. I just bought a 'Taurus' at a Home Depot a couple of weeks ago while I was in L.A. for the ABS National Convention. They also had GAC (great American cities) varities as well.

Local nurseries would be a good option too. I just picked up two rexes at a local nursery last week - 'Peace' and 'Purple Curl'.

For online sales, try "Plants for Kids". They sell GAC and constellation series (Taurus, Capricorn, and Libra). Rob's Violets and Logees would be two more to check out. eBay has them from time to time so keep a check on that site daily or weekly. There are other online sites but they don't cater to rexes as much as other varieties.