Have you tried local nurseries or garden centers that sell fruit? Like most fruit, grapes should be specific cultivars recommended for that location so mail order sources - which could be across the country - are not always the best choice. Local will always be larger plants as well without shipping considerations, like dormancy, size and being bare root.

Generally grape vines sold will not be very mature......that's kind of an issue with any woody perennial vine. They produce big root systems as well as large woody trunks/stems so mobility and therefore size will always be limited. 2 year old plants are the most common or those pruned back to 2-3 buds. And as is the case with a lot of other vines, these need 3-5 years to settle in, develop a good root system and start to produce heavily.

I buy my grapevines from Stark bros. and have never had a problem. They used to sell them bare root, but have switched to selling them in pots. I bought two Thomcords this spring and they are doing very nicely.
They also know which varieties to sell, and ask for your zip code on the site so will match cultivars to your growing region.
gardeningal is right about maturity and the size you will find online. The Thomcords were about 12" vining, and will take several years for me to have good fruit-set. So if you can find a good seller locally, you'll end up with bigger plants that will produce good crop size sooner.