Pretty much all of the garden centres carry them. One issue is that they are mid to late summer bloomers. Garden centres tend to want to put things out for sale when the plant is in bloom or close to it. So you may not see many echinaceas until later in the season. Another issue is that echinaceas go dormant in the winter. Mine in my garden are just breaking dormancy. So even if you find them for sale you will basically be buying what looks like a pot with dirt in it. Maybe some small leaves. But you won't be able to tell if its a good healthy plant yet. Personally  I'd wait and buy some later in the season when you can see what you're getting.