I just picked up a HD Sportster and would like to add a larger gas tank so I can get 125+ miles on a tank of fuel.  I don't plan on using the Sporty as my touring bike, but there are quite a few rides out in the desert where you might not see a gas station for 100 miles or so.  Does anyone know of an oversized tank made for the Sportster without having to do a lot of modification?

I gather you didn't get one with the 4.5 gal tank. I also know someone with a fatbob tank on their sporty. Don't know what they had to do to mount it though. I can find out if you like?

You can look on line for the Vtwin catalog and they have assortment of replacement tanks for sportsters as well as other harleys. Most dealers and local bike shops can order anything you find there. There are a few that will do it online as well. This is how I got most of my parts for my old 76 Sportster.

My sporty came with a 4 gal tank, it is a 2006 custom, was not changed and i get 175 miles before i need to be worried. Dont know what year you have but, this should do it. can tell you a 99 sporty tank wont work on mine, frame on mine is larger. Hope some of this helps.

It seems to be just over 3 gallons.  I got to reserve the other day and was only able to put 3 gallons in.