Assuming you mean 'Benelli' shotguns... type benelli into any search engine and you'll find their official website.
Look on auctionarms.com or eBay
Try Blue Book of Gun Values
there are many avenues to find barrels for A5's. Try the "gunbroker.com" firearm barrel section. A5 barrels are sold there all the time.
Yes
With a break open shotgun, the part of the gun frame that is hidden by the closed barrels.
A top break revolver
it depends on the manufacturer and the condition of the gun. it helps to know the caliber of all three barrels. usually you have two shotgun barrels and the third is a rifle.
15-65 dollars
Call Ithaca. If they don't, they might know who does.
Steel or Damascus barrels?
The 940 was a single shot shotgun. Sounds like it comes with 2 extra barrels. Values range from 50-100 for a complete gun. With the other two barrels, depending on condtion, you would be in the 75-175 range.
Can't be answered without a detailed description of action, stock, barrels, markings, etc..