What's wrong with the one on the gun? If it's stricktly aesthetics and not physically damaged beyond the point of being safe to shoot, they are very easy to repair. Use a chemical stripper to get rid of the old finish, not sandpaper. After the old finish is gone you can use an iron and a wet towel to get rid any dents, then lightly sand down to 220 grit, no finer. I use Formsby's low gloss tung oil finish on all of my guns, $8 a quart at lowes and it's very easy to apply. You apply it by rubbing it in then wiping it off, two coats a day for about a week and you'll have a gorgeous gun again.
On the other hand if the one on the gun is damaged, check out this one.
http://cgi.ebay.com/Marlin-336-Stock...item27abb8d32d