You should be able to get a Mossberg "youth" stock (12-13" LOP) that will fit a 20 ga. with no problems -- either from Mossberg, or used, from someone whose kid is getting bigger and needs a full-sized one. The wooden stocks are inexpensive. Mossberg also makes shim kits that let you adjust both the LOP and the angle of the stock and receiver -- a custom-ish fit, for not much money, with a traditional-looking stock. The AR-15 style stocks aren't ideal for a shotgun, especially for a new shooter. They tend to bite the cheek, which is discouraging for a beginner: it's not supposed to hurt...
Call Mossberg (800-363-3555)and ask them what they have. I think you can get a new one for $60-ish; less if you can find someone who's moving to a longer stock and wants to sell a used smaller one.
Never let anything mechanical know you're in a hurry.
Last edited by Evan Thomas; February 14, 2013 at 10:16 PM.