I suggest Double A Vineyards. I was extremely happy with what I received last year.

Here is a link that might be useful: CONCORD

I live in Northeast Ohio and planted some Concords and Concord-types this spring. I ordered from Schlabach's and Stark Bros. All the grapes did fine this year, and one (a Van Buren from Schlabach's) had a few bunches of grapes already. Indianaberry.com also has them. Schlabach's doesn't have a website, you have to order a catalog through the mail and send 2 dollars (I believe they are Amish). They don't send the new catalogs til after Christmas. Address is 2784 Murdock Rd.; Medina, NY 14103. I posted the grape page from their catalog a few months ago on the following link.

Here is a link that might be useful: grapes

I would second the vote for Schlabach's. I planted one each of Reliance and Marquis seedless, and one each of Swenson's Red and Concord Seeded this past spring, they all grew nicely. The Concord grew the best, and reached the top wire of my trellis set at 6.5 feet. It is known to grow well in my location of central PA, and I expect to get grapes from it next year.

Hi I really like Double A vineyard. I have a five year old Concord grape that produces abundantly, however, the birds and coons get to most of the fruit before I do, even though I cover each cluster in beige cotton bags. Its a trial.. I will have enough grapes this year for one Kuchen! Mrs. G

The vines I have received from AA vineyards have consistently been large and extremely well rooted compared to other sources.
That said, within a few years I can't tell the difference from those vines and smaller ones sourced elsewhere.