I am putting in one from Eurocampers.com and like it so far.  You could also
check out

http://www.plastic-mart.com/class.php?cat=21

I can't vouch for them or anything, just found the link when I was looking.

>From: David Wilhite <dlwilhite@COMCAST.NET>
>Reply-To: dlwilhite@COMCAST.NET
>To: vanagon@GERRY.VANAGON.COM
>Subject: Under Floor Water Tank Still Needed
>Date: Sun, 21 May 2006 13:25:54 +0000
>
>Guys,
>
>I am still needing a water tank after changing my Westy to a 7 passenger
>keeping the stove and sink intact.  My preference is to put a tank under
>the floor, but I have been having difficulty getting a tank I can afford
>that will fit.
>
>I have seen the Reimo website, but from what I understand the shipping
>costs would make it cost prohibitive.  Eurocampers carries one (~$95.00),
>but they don't have any in stock and they may or may not get some soon.
>
>There are plenty of polyethylene water tanks on ebay at good prices
>(~$40-$80), but none of the standard sizes will fit.
>
>Of course, I can get a custom made friction-welded tank for $400, but
>that's way out of my budget.
>
>A final option would be to pull the fridge and install a water tank there.
>I do not want to take this route, because I do use the fridge occasionally
>on camping trips, but I may have to.
>
>Anybody have any other sources are some other ideas of how to add a water
>source?
>
>Thanks,
>
>David Wilhite
>84' Westy
>Middle TN