There are plenty of "AR" type stocks that are collapsible (6-position) for not much money. Myself, I don't like those type stocks with a pistol grip on a shotgun, especially a Mosbberg with the safety located up top.
For regular style stocks... (I'm linking straight from the manufacturers sites, you can most likely find better prices elsewhere. I'll leave that up to your own google-fu)
You can try a 12" LOP over molded stock from Hogue. I have their standard length stock (14") and think its great. Hogue's site says they should fit 20 Ga. no problems. (I know you can find it cheaper if you look around.. normally about $45-$50)
There is also a place that sells a 13" LOP Hogue stock (only place that I've seen have them). This would be in between the regular 14" LOP and the 12" LOP linked above... the middle ground, so-to-speak. Only issue here is, you have to buy it as a kit that comes with a forend that most likely will not fit a 20 ga. So, the stock would fit, but you may end up with a unusable part along with it.
Or there is an adjustable stock from ATI called " Akita" made for 20 ga. Mossbergs. It adjusts from 12-3/8" up to 14-3/8 for a wider range of fitment options. Also has a nice cheek rest on it. A bit pricey though.
Hope this helps somewhat.