A lot of the pros use 30 & 50 gallon drums for floats because they're cheap but strong. I use 5 to 30 gallon plastic drums for all of my floats. There was a place on line where you could buy manufactured ends for the drums also to give them the curve if you need to stack them end to end for large barges and so on. Plastic or steel, both work fine for different situations.
OP, My guess is they outsource them locally. Once they have their design they order as they need. Shipping is quit a bit of money based on the bulk.