I've got an old 500 sitting around that has a busted plastic assembly, among various other problems it has.  No one *ever* has the metal 590A1 trigger assembly in stock.  Anyone know here I'd find one?

That is a unicorn horn.

Brownells shows back ordered, i assume you looked there already.

https://www.brownells.com/shotgun-parts/trigger-group-parts/trigger-guard-parts/trigger-guards/trigger-assembly-prod24369.aspx

Yep.

I'm thinking repairing this shotgun at this point is a lost cause, economically.  It needs a new trigger group, which is $100 even if I get the craptastic plastic one.  Needs new furniture, that's another $100.  All metal needs to be refinished, no telling what that costs.  Another $130 or so for a new barrel, assuming I can find one I want (wanting a cylinder bore with bead sight).  Also want a full length mag tube, another $40 assuming the 590's tube fits a 500.  I'm in it for $370 minimum at this point, not counting refinishing cost.

Truth is I'd be better off parting out the good parts and putting the money towards a new shotgun.  LGS has a used very good condition 590A1, complete with lug for the pig sticker for 349.  That might be the way to go.

I'd fix the trigger guard and make it functional. Then either sell it or hunt with it. Put your money toward the 590 you want then.
There's a bunch of the plastic trigger assembly on eBay in the $50~ range buy it now option. Used 590s run $300-400 in my area on armslist and such. You're better off having a beat up meat gun and the 590 you want rather than cobbling something together.

Sounds like a shit show to make the 500 you have "right". If there's no sentimental value, I vote to ditch it as well.

Afaik all 590s and 500s have their own style of mag tube/ barrel retention. My 500 (20ga) has a captive screw for the barrel, while my 590 has a cap nut. The 500 mounts on that screw, while the 590 slips over the mag tube and is held on by the nut. The best I can remember all 500 and 590 use the aforementioned system, respectively.

I want to believe you can swap the mag tube out (can be a PITA) and convert one to the other's style.

Feel free to correct me if I'm wrong.

Still, totally not worth it. I'd unload it and make known the trigger group is fucked. Maybe pick up a combo package with the short fixed, longer screw in choke and rifled barrels. Seems like a deal to me, I saw one in a camo dip for like $375 in a pre-hunting season sale.

No sentimental value.  I think the furniture and finish is jacked because someone attempted some bubba gunsmithing and wasn't able to finish the project.

I'll probably get this one in good enough condition to shoot.  Recoil pad, plastic trigger assembly should be enough...might rattle can paint it.  Normally I don't care for doing that sort of thing, but this is a scenario where that's actually a viable and appropriate fix.  I figure I can do all that for about $125 and since the barrel takes chokes, I'll have a bird gun that I won't care if gets wet, beat, or banged around.

I'll save my pennies and buy a used 590A1 for my anti-intruder gun.

I would buy the Mossberg 590 A1. If you feel like using it for hunting you  can add a barrel from a Mossberg model 835, that's what I did, works like a charm. In fact it's the only shotgun I own.

I'd like to know how someone managed to break a plastic one. I've been fairly rough to my 500 and have yet to even scuff the plastic.

Just place back order with Brownells. Took about a month to get back in stock for me. Mossberg won’t sell to you if  you aren’t a gunsmith but told me to get it from Brownells.