If you have a lawnmower/snowblower/snowmobile junkyard in the area, they will probably have something for you. I've got a little tank from a late 60's mower, it's orange in color and I don't recall the brand - but it's not a common name (at least not any more).
I use it for tuning motorcycles; you need to take off the tank to adjust most cycle carbs, and this little tank works perfectly as an aux. fuel supply. Even has a little needle-valve petcock on the bottom.