In a bigger city you will find home wine making clubs. They are a good source as some of them order in truck loads of different grapes from places like California and Washington.
Some larger Local Home Brew Shops (LHBS) also will order large volumes of grapes in season.
If you have the bucks, you can also order in buckets of frozen grapes, which make some pretty nice wine. Brehm Frozen Grapes is one such.