I don't think a 3 1/2" chambered 18" barrel is available for the Super Magnum.
However, you can use any 3" chambered barrel, you just won't be able to shoot the longer 3 1/2" ammo.
Sources for 18" barrels are :
Direct from Remington.
Gun Parts Corporation--
http://www.e-gunparts.com
Brownell's--
http://www.brownells.com
MidwayUSA--
http://www.midwayusa.com
Cabela's--
http://www.cabelas.com
Ebay.
For a magazine extension, Brownell's sell most of those that are available.
A good brand for price, quality, and choice of finishes is the Choate extension.
The Choate is available in gray-black parkerizing, and in a satin black sulfate finish that should match your gun.
Best of all, the Choate comes as a complete kit.
You get the steel extension, a high-grade long magazine spring, a orange plastic heavy-duty magazine follower, a sling swivel, and the all-important barrel clamp.