I've got a bunch of those little single serve jam jars, glass jar with metal screw-tops.

Traditionally, I know folks use the corks, but were metal tops also appropriate technology for an adventurer?

Or is this something for Eberron or the Krynn Tinker gnomes?

Good question. I always use whatever is most leak-proof but when I use screw tops I melt wax over the top to make it look corked and sealed. 

Historically, cork-stopped bottles would be the most appropriate if one was shooting for a fantasy based historical appearance; however, if availability demands, using a screw top and explaining your way out via a Tinker Gnome's technology would be okay. 

A good source for stopper bottles can be Micheal's craft stores, and some quality estate sales. 

Historically, bottles could also have been leather with bone or wood stoppers as well. Most traditional wine skins and canteens were made in this fashion. Hope this helps!

For me, I would avoid metal stoppers.  I would favor cork stoppers for finer quality pieces and wooden stoppers for lower quality.  Anything like a potion or very fine wine would also be stoppered with a wax covering.  A jar as your describe might have a wooden stopper, possibly tied onto the jar, in many instances.