I got my Jumbos from Freshops and Steinbarts. Its been a few years back, but I think you can still find the Jumbos during rhizome sales time in mid-march. I think I paid around $8 for them, as compared to $4 for the regular rhizomes. The ones I got for Willamette and Cascade were really big, almost root balls and actually gave me a decent harvest year one.
You won't be disappointed with great lakes... Me and a buddy bought 50 crowns (5 varities) for $460. Can't wait to see what they produce in the summer!