ORIGINAL: DarZeelon
Bladder type fuel tanks are not common enough yet.
Currently, most are designed for racing and are made by (for) racing engine manufacturers, like Jett and Nelson.
Tettra also makes them.
Most of those offered are of small capacity (4-6.5 oz.) and carry a price 4-5 times their normal, same capacity equivalents.
I guess maybe by the time they're mainstreamed into the rest of the RC community they'll be a little longer lasting and less expensive. I'd pay $20 for a 12 oz tank that's foam free, no possibility for fuel contamination, and not having to worry about the clunk getting clogged up.