Caradonna is definitely a great salvia - I seemed to see around quite a bit last summer. I know Gertens in Inver Grove Heights carried it last year and it's in their catalog for this year. I would guess Bachmann's and Linders have it as well. They probably are going to want about $10.00 for a one gallon plant. If you can find some that overwintered from last year they could be divided. Black's in Afton sells a pretty broad selection of perennials in 3" pots. Bluestone perennials offers small plants by mail orders. Either that might a more cost effective option.

Good luck

Ted

Winter sowing/spring sowing from seed will give you many more than 20- if you want to go that route- I have sown my seed already, otherwise I would have shared- but a request on the seed exchange forum - or a google search for seed will give you more than you want for quite a small amount of money.
Sowing outdoors in "protected" pots is very simple and easy- and I do believe it will bloom first year from seed.
I have sown it this way before- and will again.
Julie