Yeah, like everyone says, look for a used m500. Check local guntrading sites, post a WTB ad on craigslist and backpage. I dont know if you go to a range with another firearm, but check the board for items being sold. If you keep you eyes open you may find gold. Especially on craigslist, just make sure you get a bill of sale if possible.
With todays market, buying a decked out shotgun is going to cost you an arm and a leg due to the tacticool tax that these sellers are tacking to the prices nowadays. Its stupid. Also, just a heads up, if your right handed be aware of the fact that with a pistol grip stock, your goning to have a hell of a time pushing the action lever. If your a left, its heaven. lol
Also a side question, ive never shot one, but how does 20 gauge 00 bucks recoil compare to that of a 12 gauge reduced recoil 00 buck?