Look right here on the classified under H&R Talk. There's always barrels for sale here, and everything you need to know for fitting is in the FAQs. That '06 should be on a SB2 frame and you can put any rifle barrel on it. A really fun and versatile barrel is the .357. Shoot .38s, .357, and if you really want to get versatile, ream it to .357 Maximum, then it'll shoot all 3.
Pete