I sold the large metal tube I found in the beginning of the game because I didn't know what it was for. I'm building a gunsmith, and am hoping there is a way to find another one, but I haven't seen any in any of the shops yet, although I'm very far in. Are there other large metal tubes to be had?

Try looking around various garbage cans/bins or go to various tech only shops. Maybe you'll find one in Tarant.

Kill the merchant and steal his stuff...