Copper is often marked as oz weight. The weight is per square foot of material. 1 oz copper is about 1.4 mils thick, so 3 oz copper will be about 4.2 mils thick. It is actually rather light for craftwork.
Roof flashing is usually in the 16 oz range (22.4 mils) and up.

I bought some of that copper roll a while back. It has an asphault like layer sanwiched between the copper and the kraft paper backing. This gives it considerably more body than just the 3 oz copper would provide alone.

I got it with the intention of using it to band citrus trees to keep snails off them. Still working on that ;-).

You can get small sheets of sheet copper at Orchard Supply Hardware. It's not cheap.

Greetings,

I know this is an old thread, but I have been trying to make sense of all the various ways copper sheeting is referred (want to use some for snail/slug barriers as well--5 mil or 36 gauge seems to work the best and be most affordable) and came across a really good thickness guide on basiccopper.com

Art

Here is a link that might be useful: Copper Sheet Thickness Guide