I've seen it in the perennial section of certain garden centres for big $$ but it doesn't overwinter here. I started mine from seed, and bought new seed this year because I didn't want to risk not having it again. But, my plant from last year self-seeded all over the place and now I have verbena bonariensis growing up between my patio stones! Not hard to remove, though.
The self-seeded ones are blooming only a week or two behind the ones I started inside this spring, so hooray! Looks like verbena bonariensis is here to stay.