From: Winnipeg, MB
avgas is a derivative of supply, in the sense that if you have supply you have avgas if you need it. I believe the rules for the movement of oil and fuel are identical, and that they can move overland however slowly.
Supply and fuel and resources flow to where the demand is. Once that demand it getting enough, the taps slow the flow and accumulate any excess at the biggest port within range/access of the flow of materials. Besides flowing over land, a small amount can flow between to ports that are one hex away from each other with adjacent traversable sides (small boat transfers).
So to get some of your other bases to hold more fuel, use more by refuelling ships or loading tankers/AOs. It takes a few days but within a week more will come in to fill the previous demand. Example - the game starts with not much fuel in LA, but I made it my tanker base and started filling tankers there. Within three weeks it held over 1 million fuel stocks and within six weeks it held over three million fuel. That's extreme because the US had virtually unlimited amounts of everything, but you get the idea.
PS - per Alfred - a base will only hold 100 days worth of demand (based on the demand of the previous 30 days) so it will not go on accumulating infinitely.
No matter how bad a situation is, you can always make it worse. - Chris Hadfield : An Astronaut's Guide To Life On Earth