My first choice would be to purchase the barrel from a local gunsmith. Just the make and model number should be sufficient to get you the barrel you need.
The largest online/mail-order source of parts and equipment is Brownells.
http://www.brownells.com/index.html I am skimming through their 2002 catalog (#55) and don't see any slug barrels for your model 140. Not to say they can't get you one.
A quick online search does show the existence of a Model 140 slug barrel. Scroll down
this page and you will find "1400--20 Inch,Rifled Sites--Slug Barrel--20 (gauge)--$165.00
Also check the listings on
this page model 140/1400 parts. There are a few.
Unless the laws have changed, you can purchase a barrel for a firearm by mail order without the assistance of an FFL dealer.
Also, I wholeheartedly agreed with what Cliptin wrote. A hunter has the responsibility to kill a game animal quickly and cleanly with no needless suffering. Using buckshot or a rifled slug in a smooth-bore barrel is just asking for inaccuracy and the potential to nonfatally wound the animal.