There are several aftermarket tanks, but most deal with limited space from longer wheelbase modifications. One is suppose to give an extra couple of gallons and bolt right in the stock location. They usually come with their own skid plates, but may use the stock pump. All this for better than $300-600!!!! If anything, do a parts search on
Car-Parts . Who knows, you may even find a tank that someone swapped out due to modifying the wheelbase. Just check the major 4x4 forums like Pirate4x4 and other large Jeep only forums like jeepsunlimited, jeepforum, etc.
If anything, I suggest that you upgrade the stock skid plate. I doubled a newer skid plate and haven't had a problem with mine on my 97 TJ. Another thing to add, you may want to mod the stock gas tank to gain a few more gallons capacity. Check out
Gas Tank Skid