So long story short, I have an 870 breacher (sku 82710). After speaking with a remington rep, he confirmed that the MCS barrels will work just fine on the gun, and that if I swapped on the qd plate, so would the MCS stocks. The problem is, after trying several times, remington's answer is "hell no we won't sell you any MCS parts, you can't own that gun". Well, my form 4 says otherwise, but I digress. I checked with the remington LE dealers for my area, and none are willing to sell me the parts. Does anyone know of any source to get them? I'm specifically interested in the 14" barrel and maybe the qd stock plate and stocks. Thanks!

Did you try Lawmen's and Shooters' Supply in Vero Beach, FL?
www.lawmens.net

Good luck, I did an SBS project where I went the non MCS route with a limited volume hunting 870 shotgun Remington produced in the 90s that had same length tube and barrel ring spacing.   Thought I could buy the MCS forend without issue, lol, its like trying to order a nuclear ballistic missile submarine.

If you find someone at Remington that can actually look up the part, hushed tones suddenly greet you over the phone as they likely try to get in touch with the NSA to report your activities.   I tried a lot of other sources and no luck.  

I finally gave up with improvised solutions and just ordered the folding forend grip from Safety Harbor although that also needed a spacer to properly work for my application.

I narrowly missed out on a supposedly off the registry MCS that was being sold with its 18" barrel on gunbroker for 500 odd bucks, I would have bought it just for parts.

The gun I used is an 870 Special Field.  They came in 12ga and 20ga.  If you can find an extra barrel for one, I believe it will fit your MCS.  However, they all have ribs and are usually high gloss blue.  Remington doesn't make much mention of them:  https://support.remington.com/General_Information/What_barrels_are_available_for_my_Model_870_Special_Field_shotgun%3F

Ebay usually has at least one barrel in 12ga although I don't see any now.  Mine is set up with a 10" barrel with a fixed breacher on the end.    I would be interested in an extra barrel to have without a breacher, but they tend not to go cheap and they you need more work on top of it.  If I had to do it all over again, I would just go with a regular 870 and chop to 12.5", alot less headaches that way.

It actually sold for 500 something dollars? That seems crazy low for the MCS.
What does it matter if it's "off the registry" or not? If it's in Title I configuration, then it can transfer as a Title I firearm.

It actually did not show as sold.  It was a listing of either short duration or it was pulled early.  Probably sold via email.  However, it was mentioned as being off the registry which perhaps scared some folks.  But, you can officially request it to be delisted with ATF.  If I was selling one, I would notify ATF otherwise you might get sucked into some criminal investigation when some jackass down the line makes changes.

Anyway, if you bought it in title I configuration, you are left with an 18" gun that was an overly short foreend.  Might as well buy a regular police instead.  And if you wanted to SBS it afterwards, then you have the same issue that is the subject of this thread.

For parts I'd call JD at AI&P Tactical. I think he's a Remington Factory Certified Law Enforcement Armorer.