The Web site is from a place in France that builds a direct bolt-in replacement, 200 litres, twice the size of the one you got now. It's a pretty popular upfit in Europe. I've got an uncle in Germany that has one.
www.acgb.co.uk.
Not much information on the site, but I know that unless you want it really, really bad, it's gonna cost more than you want. The sea cargo freight alone is gonna run $800-$1000. That'll put the total cost upwards of $1500 by the time you pick it up at the docks.
Transflow should be able to come up with something, or jegs.com should. You'd think that an aftermarket tank for the Sprinter would be not all that hard to find at this point. But it is.
Slow and steady, even in expediting, wins the race - Aesop