15's are going to give you a MUCH wider choice of tires. What size tires are on the Durango now, and give me some ideas of what kind of spacing you have (tire to fender, inside of tire to suspension/steering, etc.). I don't know much about Durango's, so someone like ritz might be able to help you a bit more since he used to have a lifted Durango, but I can give you a fairly broad estimate based on the info I asked for above.

And any local tire shop can get your basic wheels and most tires.