Many places sell them. Depends on what you want.
Check the TDR magazine for ads, or do a google search.
I put a 45 Gal in-the-bed Aero Diamond Plate tank in my '95 a couple of years ago. I love it. I've probably saved a couple of hundred dollars in fuel money by being able to fill up at the truck stops instead of smaller, more costly stations.
I can go about 1600 miles between fill-ups if I need to, but I generally fill ONLY the Aux. tank and then transfer it to the main. I have a big NAPA fuel filter/water separator on the Aux. and so most of my fual gets filtered twice. It's good insurance.