Looking for a barrel...

About  40 years ago I bought a Topper Model 158  20 guage single shot.  Made by
Harrington & Richardson Inc. Worc. Mass. USA
At that time I should have gotten an extra barrel for it.  Now I'm looking for a  243 or 30-30 or a 30-06 barrel.   Where can I find an extra barrel ??  Do they still make them?  In todays market what would be a fair price for an extra barrel ??
I would like to give my grandson this gun for small game and deer hunting.
Thanks for any info you can give me.