A few years ago I made a small boiler using some 10mm diameter salvaged tubing from a central heating system.
After the boiler was made and when pressure tested one of the boiler tubes split along it's length, rendering the boiler useless.
Since then I have always used new tubing.
If you have a local car brake specialist in your area, they should be able to order in (or have in stock) small sizes of annealed copper tube for very realistic prices. I would buy it in 5 or 10 metre coils.
If you don't try it, you will never know if you can do it!!