I've had some very good fittings from Stuarts, PM Model Engines and Steam Fittings UK. A lot depends on what they need to look like, being rather prominent on the sides of the boiler.
Thick walled copper tubing should be no problem if not too long. The best bet would be to visit a supplier near to you, they may be able to cut pieces exactly to the length required and then you could take them home yourself.
My only experience with having fragile materials delivered through the post is with some items obtained from Macclesfield models who strapped them to a piece of wood to ensure that they arrived undamaged.
Brian