Check this out. We5inLgr recently put together a growing list of sites and vendors. Lots on there.
Links To Online Sellers Of Fresh Grapes, Juice & Frozen Pails For Delivery
If you want to make a four hour trip, Harford Vineyard (north of Baltimore) gets juice and grapes in Spring and Fall. You've probably missed your window on juice, but there may still be some grapes coming in that haven't been claimed.
There's also S&S Grapes south of Baltimore. I know one person that uses them, though I never have. I've called and emailed a few times with inquiries over the years and have never received a response. I guess business is really good for them.