Hi Phil,
I purchased a length of copper tube from a company called M-Machine in County Durham. They have a website –
www.m-machine-metals.co.uk – but you cannot purchase off the site. They have listed quite a few odd sizes in a pdf catalogue and I’ve just checked, 3 1/2” x 10 Gauge is listed. You need to phone them and pay by card. They were very helpful and will supply any length you require. Their phone number is 01325 381300.
Michael