Kinda like a futures contract I suppose.  Can you, as an individual buy 500-1000 gal of gas?

I never heard of it for gasoline, I can pre buy propane down here.

Gasoline futures are traded on CME, NYMEX, and ICE.  If I recall correctly the standard contract size is 1000 barrells (42000 gallons) settled in cash or physical delivery.  Less than 2% of futures are taken as physical delivery.

So the short answer is no.  Futures and options are mostly used as hedges.

Just purchase from a company that makes fuel deliveries.  Since you're in Texas like me, I know that Suncoast is one such company.  I don't know if they would do business with an individual or if they just do business to business transactions.

I buy 1000 gallons of diesel at a time about five times a year. They deliver it to my house and I store it in a tank.

If you have someplace to store it sure you can buy however much you want.

My ex father in law was a farmer.  He had a tank and bought hundreds of gallons at a time.

They are called Fuel Banks. Enter into a contract to purchase X amount of gas at a certain price. Use it when the price of gas goes over your contract price, and buy retail when the price of gas goes below what you paid.
Only available in certain places. Place I'm thinking of is First Fuel Banks in the Minneapolis area.

My dad has a large excavating company and does something like this for gas and both off and on road diesel.

He just pays them whatever the price is at the time and they give him whatever he orders.

What about NOT taking delivery?  If gas prices fall, I'd like to pre-pay for 1000-2000 gallons.  I know you can get pre-paid fuel cards, that's just like any other pre-paid card.
It would almost be like buying a futures contract, but with no expiration.  I dont think this exists.  For now, you want to buy a bunch of fuel to take advantage of low prices, you gonna have to store it somewhere. Also, most gas stations only allow up to a set $ amount at the pump. So you would have to make multiple trips.  Then there is the little issue of... safely storing that much gasoline in one place.  An extra 20-50 gallons I can handle, but 1000 ?  I think Id rather just use the funds instead to pick up additional shares of oil stocks.

So you want to pay Exxon for 2000 gallons of gas today at a reduced price and then pump it 20 gallons at a time until you used the 2000 gallons?

While that would be nice, I doubt it is realistic.

IMHO, yes and no.

Some have indicated it's not possible, but I will disagree. That being said, there are some stipulations.

You aren't going to find anybody doing a futures contract for 20 gallons at a time twice a week...

You most certainly can find somebody willing to do futures contracts for ~500 gallons and up on a recurring or one-timd basis.

IE, my dad runs several businesses and uses thousand of gallons of diesel per year (maybe tens of thousands in a busy year). At one time he was able to buy fuel through the local co-op on futures contracts (this was bulk delivered fuel). The price was based on the large market futures prices plus the mark-ups to get it to the customer (IE, the co-op has operating costs, they aren't buying fuel on the exchange and selling it to you for the same price). I know he isn't buying fuel that way anymore because his nature of work wasn't predictable enough to solidly lock-in consumption amounts. Guess what happens when you have a contract for 2,000 gallons of diesel but only have 1500 gallons of space in your tank. They aren't happy; a good co-op will work with you, a not-so-good co-op will still make you pay up and you forfeit the fuel that didn't fit. I'm not sure if he quit buying that way because of the unpredictability or if it's no longer an option at the co-op he uses. That being said, you SHOULD be able to find a bulk fuels supplier that services your area willing to do this. I can't say what their terms will be, but they may want you to take a full semi load at a time (IIRC, ~8,000 gallons).

On a side-note, dad also uses lots of LP and buy by the semi load. LP is really cheap by the semi load... like sometime half the price you're paying to get your 500 gallon tank filled...

South West Airlines does.

Yes you can. For the farm we contract diesel and gas. Now we do 5,000 gallons at a time. Some times a lot more.