Cheaper than dirt, Midway, Cabela's, Bass Pro ... Anyplace that carries sharpening stones. The pocket-sized stones will easily fit between the rails.

That said, IMHO, all that polishing the inside of the rails does is reduce some minor bit of noise. The bolt is not a tight fit on the rails by any means and polishing, again IMHO, will do essentially nothing to change performance in any way, including bolt speed on recoil and returning to battery. Personally, keeping the rails and the slots in the bolt body clean and properly lubed is all that's really necessary.

Even though I did polish the inside somewhat, I think it's one of those "feel good" things - if you think it makes your rifle operate better, have at it.