I live in Southern California. I have an easy time finding websites and sources discussing buying wine grapes by the ton. I even found 1 source that will sell a minimum of 100 pounds. However, I cannot find any sources for buying fresh grapes in quantities less than that. Does this exist? How do folks find local fresh grapes for homebrewing?
I'd check craigslist. I poked around on some Southern California CL pages for "grapes" and saw some entries with pick yourself pricing for folks wanting less than 100 pounds.
Granted, 'Southern California' is a big place- I have no idea what region you are so I can't verify, but it's worth a check.
In the UK (south east) there are many orchards, fruit farms and PYO (pick your own). The best way I found is if you grab a couple of bottles of your last brew and visit a farm on the off chance, if can speak to someone important (may need a second visit) and explain what you are trying to achieve and that it is for personal consumption. A couple of bottles generally goes a long way, also with a promise of a couple more bottles made from their own produce once it is ready.
I already do this with a hop farm, and a pear farm. Next year I will be doing exactly this with bee keepers, as I would like to try making a mead which will need about 30lb (14Kg) of honey for a couple of batches ;)
I have found that many farmers produce their product and sell it for profit (duh) so they don't really have time/effort to ferment it, so if you can give them something back they really do appreciate it, more so than the actual $$.
Or another alternative might be labour in return for grapes, you never know you may learn something too!
My LHBS sells them by the crate during harvest season. Each crate is approximately 36lbs of grapes. It takes 2 crates to make ~5 gallons of wine.
My LHBS doesn't ship the grapes so I can't help you there. Its for local pick up only. I only point out that maybe you can search local homebrew shops around you to get something similar in quantity.