Hi Pooney67,
the best thing to do is to google[or which ever search engine you use] the plant by name with 'to buy' after it them it will suggest garden centres in your area. I'm in the UK so cant advise.

I came across this website whilst researching something else, maybe you could try here as a starting point

http://www.drought-smart-plants.com/mail-order-plants.html#axzz2GYeerUR9

This link takes you to the US mail order page but it doesn't give you very much information, but it's always a start.

Hope this helps a wee bit!