Took me a while to find what was the closest thing to your area's local Farmer's Cooperative, but I'd try calling these two stores and asking them if they carry "Agricultural Vermiculite in 4 cubic foot bags" and if they tell you that they don't, ask them for a referral where you could find it.

Keep in mind that sometimes the guy/girl answering the phone is making minimum wage and doesn't really know what they're talking about so if they try to brush you off just ask to talk to a manager. These stores are built around ideal of farmers helping farmers so it might take a little legwork but one way or another they'll get you the product - or at least the information - you need.

Agland, Incorporated
Murdoch's Ranch & Home Supply

(for what it's worth, a reasonable price for a 4 cu. ft. bag of agricultural grade vermiculite should cost about $15-20 a bag with a decent price being around $17 USD these days)

I found it a nursery, but my local Home Depot just got a big pile of huge bags... check there or ask if they will order it...

You may want to ask about 4 or 8 cubic foot bags. I got lucky and got an 8cf bag from a local nursery (for around $27). When I asked for the 4cf and they said they didn't have them the knowledgeable employee I was talking to asked if I would be interested in the bigger bag. Dealing with a less knowledgeable person I probably wouldn't have heard about the "big bag" without asking about it specifically. These big bags may not be very common, but I wasn't about to question my luck!

American Clay Works in Denver carries it for $13 per 4 cuft bag. The number is 303.534.4044

for what it's worth, a reasonable price for a 4 cu. ft. bag of agricultural grade vermiculite should cost about $15-20 a bag with a decent price being around $17 USD these days

I found it today at a local garden center - the one people recommend for anything in the least way unusual - for exactly $17.

From garden web.....................You can get vermiculite in the Denver area at American Clay Works. 303-534-4044. They are a wholesaler to local greenhouses but will sell to the public. I bought some coarse vermiculite for $12.70 a bag versus $20 at the garden centers.

Okie

Thank you so much to everyone who answered my inquiry today. I now have a few sources to check out in my search. (I really did not know where to start). This has been very helpful.

Hi, I live in Dutchess County, New York, and after a very lengthy search, I found Vermiculite in a small local nursery, paid $27.00 per 4 cu.ft bag. Their 2 cu.ft bag was $17.99.
So, since I could only get 3, (that's all they had available), I'll be using only 1/3 of the required vermiculite for my size SFG, and replacing the rest with additional compost and peat. Hopefully I can add the rest of the vermiculite next year.
It is very hard to find 'coarse' vermiculite locally, and outrageously expensive to have shipped if purchasing online.