Most gas stations do sell gas cans in the automotive aisle.
If you go down the aisle where oil, antifreeze etc is sold at the gas station you can usually find gas cans in the gas station for sell as well.
Not all gas stations will sell gas cans but most of them do.
The gas can will usually cost more at a gas station than at a place such as Walmart though.
Gas stations do make some money from selling gasoline.
However the profit from gasoline is not all that much and most gas stations only make around 3 to 20 cents per gallon off of gasoline.
The more gasoline they sell the more money they will make but an average small gas station owner may only make around $500.00 to $1,500.00 per month profit off of gasoline.
But even smaller gas stations may only make $200.00 per month or so off of gasoline sells.
The real money is to be made selling snacks and other merchandise in the gas station store.
Gas stations do not water down their gasoline.
However moisture and water can get into the gas tanks through leaks or through the filling cap of the gas tank when the gasoline storage tanks are being filled.
Also you might get a bit of water in your own gas tank on rainy days or high humidity days as well.
If you do have some water in your gas tank and it's only a bit of water in the gas tank then you could pour a bottle of HEET fuel additive in the gas tank when you fill your gas tank up.
That will help remove small amounts of water in your gasoline and help your engine run better as a result.
I always add a bottle of HEET to my gasoline tank every second time I fill the gas tank up with gasoline and it does really help.
Adding a bottle of fuel injector cleaner also helps to clean the fuel injectors.
If the gasoline stations watered down their gasoline then the gasoline would not work properly in your vehicle and they would soon be out of business.
Gasoline stations make barely anything off of the gasoline and make most of their money on sales in the store.