RV holding tanks are available from many suppliers in many sizes. I find it odd that a similar size to your present one can't be sourced.

If you Google RV water tanks, you will find several sources. They drop ship and can have one at your door in less than a week. Make sure when you order one, that it has the same sized holes, placed in the same location, as what you have now. On an ABS tank, these ports can be modified, but with the polyethylene tanks, they are only a factory installation.

http://www.plastic-mart.com/class.php?cat=34

Maybe none of his suppliers carry that size.  But I bet somebody out there does carry that size.  Look at the link that gard gave or google RV holding tanks and search them.   If you can't find the size you need call Outlaw.  They are buying that tank from somebody.

If all else fails, maybe you get the next size up or down from what you currrently have. There is no law that says you have to replace the broken tank with the exact fit. The underside of a LQ is pretty forgiving about that stuff. As long as you can get the openings in the tank in the right spot to catch the waste.

I believe that Outlaw uses a tank that is built to their specifically for them. I know they will ship them out. I had a friend that replaced theirs. Give them a call.