Typically you can pre-order these starting in January or February. I ordered my 3 rhizo's through Midwest supplies last year (Cascade, Williamette, Hallertauer). Most shops will only give you rhizo's from female plants that produce hops, so you shouldn't have to worry about getting a male (this is why you don't buy the seeds). My cascade and williamette both got to be 15-16' tall in their first year and produced 1/2-1 oz of dried hops. I'm expecting much more next year. The Hallertauer was planted a little too deep, so by the time I replanted, it sprouted but only got to be 6' tall. Hopefully it comes back this spring.
Cascades seem to do well for most people. Centennial is another good one (think Bells 2 Hearted). I'd choose one of those two. Try and get the other one for a different beer style so you have some variety (english, noble, or german).