www.dutchgardens.com Dutch Gardens is good for bulbs and tubers - they get some negatives on Gardenwatchdog for plants (I've never done plants from them) but generally rave reviews for bulbs. They have a good selection of dahlias including Bishop AND they have a $25 off $50 going on right now (first order of season, not repeats). (You may have to sign up for their email newsletter to get it.

Any time you want to find a good dahlia supplier try the Big List on the Colorado Dahlia Society site. Also lots of good info on dahlia care & culture.

Here is a link that might be useful: The Big List

If you are looking for just that single variety then you can also check the big box stores. I picked up Bishop of Landaff from the local Home Depot yesterday.

Availability will vary: I went to my HD yesterday and no Bishop (sadly: loved it last year)