Probably Ballistic Fab here locally. Or you might try Santa Rita or Superior steel not sure if they stock DOM though. Luckily I work for a place that orders steel and aluminum all the time so I get it a little cheaper.
check ebay too. Just watch out for shipping prices. Also try poly performance.com they have that 1 3/4 for 10.00 dollars a foot. Then again you will have to pay shipping.
Don't know what they have since they haven't yet returned my call but there is somebody advertising tube fab on tucson craigs list called 4x4 fab (520)4401248