Dave,
I have a 7 1/2" piece of what is supposed to be 3/8" tube. The ID measures about .0015 over size at .3765". The OD measures .406". You are welcome to it if it will work for you. Just pay the postage of $4.00 USD. If you want it I suggest you supply a mailing address via the PM system.
On this side of the pond the hobby and craft stores carry an assortment of brass and aluminum tubes that start under 1/8" ID and go up to around 5/8". Each increase in size is a slip fit onto the smaller one. Pieces can be soldered together to increase wall thickness. Gives a lot of flexibility to modelers and craftsmen. I mention this as a way of letting you know what is available should you need any other size. Cost varies by size but they are not expensive.
Joe
BTW, Square tubing, in sizes that can be telescoped, is also available.