While I use an aluminum transfer tank to procure fuel and store it in my barn, I use 55 gal plastic drums as fuel tanks for my generator. If I remember right, they had several sizes of these drums from 15, 30 gal on up to 55 gal. I ordered mine from Gemplers:
Closed-head Polyethylene Drums | GEMPLER'S
Grainger also sells them, as does Rural King and a few other places. I ordered a couple barrels at once since it made the shipping more reasonable.
A lot of people find these on Craigslist, but I had no luck -- I was looking for a barrel with threaded caps on top.
Be warned, these are not approved for transfer of fuel over the road, so you could possibly get ticketed.
Also, be wary of putting a hand pump on a plastic drums. The threaded cap holes are pretty thick, but I doubt they'd hold up to repeated cranking or pumping of a lever on a manual pump. A crank pump would be less stress on the drum than a lever pump, but the crank pumps generally have a lot lower flow rate. I put a lever pump on my aluminum tank, and it will do 0.5 gal per stroke. That means for every complete out-in stroke cycle, I move 1 gallon of fuel.