When one thinks of grape juice, the name concord may come to mind. One variety of grape that has a similar flavor to a concord grape is the Niabell table grape (table grapes are grapes meant for eating, not wine making). This grape is often called the "California Concord". They grow the best in mild climates, so they do well in the San Joaquin Valley of California.
The Niabell grape was developed by Harld P. Olmo in 1942. It was made by crossing the Early Campbell and Niagara varieties.
My Experience with these grapes
Grape Flavor (Rating Scale 1-10): 8
Overall Feeling: Niabell grapes pack a lot of flavor and are very sweet. They have seeds in them which can be difficult in eating. I never came up with a good strategy for eating them. I think they are good to use to make things like jelly. I am planning to use them to make grape sorbet. They are also good for making juice since they are so similar to the popular juice grape - the Concord.
Availability: They are available during the months of July and August (maybe more if you live in California). I found them at Whole Foods Market as well as Plum Market in Ann Arbor, Michigan. They do not do well in cold storage so you need to use them quickly and this is why you won't find them much longer after their done harvesting them for the season.