The best Gas Can is the ALL Metal NATO gas can. It will not leak. The old style WWII ones with the wide mouth leak, especially under pressure.
In my opinion Plastic cans should be avoided. The plastic can attract static electricity which if sparked can be a big problem. This is usually true with the cheapest ones.
Here is a link to a place that sells the NATO cans and all acc.
Nato Gas CansThey are $50 dollars, a lot cheaper than some of the other sites that have been linked here.
When you are carrying 40 liters of gasoline in the back of your truck is not the time to cheap out on cans that can leak or spark.