Ed Brown, Les Baer, SVI, STI and just about every Custom 1911 Builder offers a 6" Model, but I have not seen any 7s. You made be able to get a 7" Barrel made to fit a 6" Slide (you'll have an inch sticking out at the muzzel) from Bar-Sto or someother Custom Barrel Maker. Good Luck!
Many moons ago, I think Jim Clark Sr. cut an inch out of the middle of a 5 inch slide and then welded it into another to make a 6 incher. Couldn't this be done with a 2 inch piece to make a 7 inch slide? Find a good pistolsmith / welder. It could be possible.
I would think that Kart would make a 7 inch barrel, for a price, I'm sure!