@clarkinks has posted a few things about seedless concord. Here is one link Concord seedless like concord should be renamed reliable!
I ordered a few Everest vines this year which is a brand new large, seedless concord-type that sounds promising. I just planted them this year and they seem to be doing fine. I bought one from Double A vineyard, one from Gurneys and one from Schlabachs. They are a little hard to find right now bit I think you will be able to get them from Double A this fall if you order early. Everest Seedless Grape - new from Cornell