I get my copper sheet from basiccopper.com. I have a link to them in the right column. They have an eBay store where you can buy scraps of copper in different gauges. That should be enought to get you started with a few pieces of jewelry. You can also contact her with special needs you might have.
If you work with a lot of sheet metal and wire, then you might want to get a sheet metal gauge. Mine has two sides, one is for wire. This helps me to know what gauge I am working with when I am writing instructions out for workshops and for publication. If it is for myself, I don't bother. People will ask me what gauge I used. I have to use this gauge to tell them.
Why? Sheet metal comes in different gauges. Some are softer and can be used for things like metal embossing. I use the lower gauges for jewelry. They are more stiff and will not bend when I wear it. I use about an 18 gauge when I make rings. Any higher and it might cut into my finger. Sheet metal is like wire - thicker as the number goes down. Thinner or small diameter in wire as the number goes up.
I usually use 20 gauge wire for jewelry. I use 18 if I want it to be a little more sturdy. 16 is too hard to work with for me.