Since you didn't supply a brand name & date of manufacture it is impossible to say. I assume though that it is an old gun out of existance, in that case your probobly out of luck. But if your real patiant do a search often on eBay. They have a lot of old gun parts going through there.
No
Yes
yes
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.
Sear replacement
Some possilbes are Wolf, Jarvis, Bar-Sto
Barrels for a Remington 870 pump are running $80 and up around here. Depends on what you are looking for.
Check the Remington website under customer service for a list of authorized repair facilities.
gun shop, gun show goto gun broker.com i got a 4 inch for a tarus 66 for ten dollar .
Try Numrich Arm/ the gun parts store. OR you might be able to snag one on gunbroker.com
Nearly all current and out of production rifles can be rebarrelled. Original and drop in barrels can often be located, but as firearms become vintage such parts can become scarcities. Furthermore, skilled rifle makers and specialty gunsmiths are able to take barrel blanks and chamber-fit them.
firstly it is spelled "THROW" secondly barrels