Hi Rik,
I have used the K&S range of copper thinwall pipe for many years, available at most model shops in the UK.
I make all my own fittings plus cones and union nuts etc. As a result, I can tailor make everything to much finer 'scale' sizes and far more 'scale' than off the shelf commercial fittings, and also at a fraction of the cost.
I would encourage anyone to 'have a go' at making their own. The results should look far better than commercial fittings.
Cheers,
Julian