Greetings from Missouri,
While not in your area, I have done a little searching for Gypsum suppliers and have some information that might give you some help. I posted recently on "Calculating Gypsum Amounts," and am still in the process of clearing a muddy pond here. I had great difficulty locating gypsum locally even though I am in the heart of a major agricultural area. I was unable to find any close source for packaged gypsum but lucked on to a farmer that took delivery on a huge bulk load (160 tons) from a barge. It has been stored outside (obviously) so it is not as powdery as the sample I did my tests with. So, I am re-running my tests with this gypsum to calculate application amounts. Back to your question though, I have some information that I got by searching the Internet that you might find helpful.
One of the major brands is Ben Franklin No. 1 Gypsum. This is produced by USG. By searching Google, or your search engine of choice, on that name, you might find some suppliers that are close to you with this product.
Another product name is Valu-Fil 650 produced by ACG/Harrison Gypsum. It comes in 50# bags I believe, and they can be reached at 800-624-5963.
Sof'n Soil has been mentioned in a previous thread as being carried by Home Depot. Our Home Depot did not carry it, or at least the people at our store told me they did not, but I did find out that Pennington Seed is a distributor of the same product and so might have a vendor in your area. They are a large seed supplier. 800-658-0410. Their closest location to me was in Western Missouri, which was a bit far to go.
Here are a couple of other names and numbers that I never pursued (since I located the barge load about 20 minutes away).
Bumper Harvest Ag Plaster is produced by Georgia Pacific. 1-888-Plaster
A company called Concentrates, Inc. In Portland, OR, has gypsum for sale, but shipping might be prohibitive.
I'm not sure if any of these will work for you, but many times, if you can do a search on a specific name, you might find a website for a company in your area. Or, you might use the telephone numbers to see if they can tell you the closest dealer. One other option that I did not have to pursue, but a nearby pond owner who is a foreman for a construction company here suggested I check with large construction firms. He indicated that he thought they used large amounts of powdered gypsum for some troweled-on drywall applications. So, if you have some construction firms in your area that deal in large, commercial construction projects (banks, hospitals, schools, etc.) they might have some ideas for you.
Hope this helps.
