From late September Until After Thanksgiving, pumpkins are widely available at any grocery store for carving or cooking.
Then :::::::::::Poof:::::::::::They are gone just like <snap> that.
My friends and I figure they are either sold for deer feed to hunters or to a canning plant that makes canned pumpkin for pie.
I have probably answered my own question, but I thought there might be a pumpkin expert or something out there.
As you have discovered, fresh pumpkins are seasonal. What you may not realize is that the pumpkins used to make jack-o-lanterns for Halloween aren’t the same ones that are grown for canning.
Given that there is no demand for fresh pumpkins (or indian corn or fancy dryed squash) after Thanksgiving there is no point for stores to stock them.
I frequently drive by a large pumpkin growing/selling farm near where I live and whatever pumpkins are left from Halloween just sit in the field to rot. They don’t even bother trying to sell them this time of year.
Don’t bother making pumpkin pies with jack-o-lantern pumpkins, they are watery, bland, and not very sweet, the pumpkin equivalent of the Red Delicious apple.
Unsold produce is sometimes given away to food banks and charities, most often it goes into the trash. That’s probably the fate of most unsold supermarket pumpkins.
Most stores toss them in the dumpster. Stores with some farmers near by, might ask if they want them for animal food? Some may take them to the plant material dumping ground. Most end up in the metal dumpster though. Pumpkin farmers feed livestock or till them into the field next spring. It’s places lthat get pallets of them in and have pallets at every store for the dumpsters I don’t like to see. All my compostable matter goes into a pile for use later.
They turn in to carriages.
Sorry.
Last year, one of the other grad students picked up a bunch of Jack-o-Lantern pumpkins for cheap after Halloween, and made pies out of them. I thought that they were quite good, at least as good as the ones made from canned filling.
And I don’t know how common this is, but the Cleveland Zoo feeds retired Jack-o-Lanterns to the elephants. I’m not sure if it’s just the ones formerly decorating the zoo itself, or if they accept them from other sources, too.
Just today, I loaded a dozen pumpkins that have been adorning the front of my house since Halloween, into a wheelbarrow.
I took them down to the back of my property, lined them up on a property line wall, and blew the shit out of them with a 12 gauge.
Cool. 
I went grocery shopping the day after Halloween and they were giving free pumpkins away at the cash registers to everyone who made a purchase. Apparently, the stores (or at least one of them) start getting rid of them any way they can as soon as possible.