Paolo,
There is no near match in US standard copper tube sizes nor could I find a 90mm metric tube, so it appears to me the most expedient thing to do would be to order 3-1/2" diameter tube from one of the model engineering suppliers in the UK. This will not be inexpensive but you are unlikely to find what you are looking for otherwise. Assuming this is for a boiler, another option would be to roll the tube from a piece of 2mm sheet copper.