Anyone knows where I can find collapsible 5+ gallon gas cans? I found 1 gallon ones, but nothing bigger than that so far.
I think you'll find there aren't any. Five gallons of gas is what, 25 pounds? 25 pounds of flexed plastic is going to bust a seam at some point and spill 5 gallons of gas somewhere.
Just get a regular gas can. There's a bunch of threads discussing all sorts of fuel containers over in SF if you need want something fancy.
I Googled fuel bladders and pillow bladders and got hundreds of hits. It looks like the smallest marine bladder is 25 gallons, but I didn't search very long.
Here's one example: http://www.marinefuelbladders.com/