I love Hallson. I have gotten nice, large plants from Chris that have grown well. He's also very communicative--if you want to change something about your order or shipping date, he responds quickly. About the only thing that might be of concern with Hallson is the selection--if you're looking for something "off the grid," you might not find it there. That said, Chris sells only the hostas that he knows perform well in their test gardens and has said several times on their forums that he won't put something up for sale until it has proven itself garden-worthy.
I've also purchased from Made in the Shade (hostaguy.com). Nice sized plants and good customer service. (Although my plants from Hallson were bigger.) Good selection.
Folks rave about Naylor Creek. I'm going to give them a try this year, I think.
And thanks for the tip on Country Boy! I'm going to have to check that out, since I'm hoping to buy a lot of hostas this year on a limited budget.