John and Doug -
I would also go with Tek-Tanks or some other manufacturer if you can't find a replacement. If you have one made, you might consider just resting it on the bottom of the bilge. When I pulled mine out a couple years ago in favor of 6 gallon port tank, it was resting on some kind of platform.
I was on a bit longer passage (comparatively longer, because we're on the great lakes) and had to motor for about 4 hours when a crew member took ill and needed medical attention. A leaky stuffing box caused the bilge to fill up around the tank, half way up the batteries (which have since been moved to the starboard locker). It worried me enough to where I decided that I never wanted to lack visibility into water accumulation in the bilge again. Now I keep 100 ft of 1/4 inch hi-test chain in the bilge (to augment the 100ft + 150ft nylon I have in the fociscle (sp?)).
Its hard to say for sure why someone would pull the tank out. Most likely reason is the gravity feed issue. However, could be personal preference, a problem with the tank, or a desire to engage in yet another dirty boat project.
Best of luck,
Jack
Bella - V2620