Where can you buy a rail gun?

You could be referring to one of four different things here:

You may be thinking of the experimental weapons system which launches a projectile along magnetic rails, rather than using explosive propulsion force. To date, no such system is in production - hence, you can't buy one, and it's doubtful a feasible man portable system will be developed anytime soon.

You could be thinking of a rifle where the barrel and mechanism is built into a benchrest, for competition in Unlimited Class benchrest shooting. Those have to be custom made. See the related links section at the bottom for a list of manufacturers.

You could be thinking of a railway transported artillery piece known as a railway gun. While there could potentially be deactivated ones for sale, it's a bit doubtful. You'd need a militaria specialist to point you in the right direction.

And some people have taken to use the term "rail gun" incorrectly to describe a firearm with Picatinny or Weaver rails. Pretty much any firearms dealer can get you one.