[Euclidean]

I'm specifically looking for an 18.5" smooth barrel for my Mossberg 500. I want to compare the cost of fitting my current gun with synthetic stocks and a shorter barrel to just buying another shotgun again.

 

[Bumper]

Try Delta Force. They have your barrel on sale for $79.99 (reg. $99.95).

Delta Force

 

[QKShooter]

New One On Ebay

Up in 17 hours...it's at $67.00 with 5 bids & shipping is $8.00
Likely to go up higher so you might just be better off buying one on sale as per Bumper but here is the auction link anyway.
Ebay Mossy Barrel 18.5" Click Here

 

[Bud White]

Could probley try Brownells or Midway Usa