I have a great deal of experience scounging for copper pipe. First, if you aren't already aware of this, standard US copper tube will be 4.125" OD and not 4.000" OD, but that may not be an issue. A for sources, there are always the on-line metals outlets which are quick and convenient bu as you've been quoted already are usually very pricey. The one I tend to use (sparingly) is Speedy Metals but they don't list copper tube at all. Your best bet locally, as suggested by Stan, is to look at plumber's scrap bins, but a residential "plumber" isn't liley to have copper of that size. Your best bet will be to find a commercial "mechanical" contractor, who does large commercial, industrial, or institutional plumbing and HVAC work. Larger commercial buildings have copper water systems (never PVC) but the thing is, you have to be lucky with timing as these days the cutoffs 24" long or less tend to get scrapped very quickly. Also you will find they are most likely to be using Type L copper, which will be fine for both pressurized or non-pressurized tanks.
I have found that I can have luck by going (hat in hand ) to the back (shop) door of a firm that I am fairly certain has the material l need and I take along the project, or a part of it, and approach the shop foreman, explian what I'm doing, and just plain ask for his help in making progress on my project. I calmly plead desperation, I have exhausted ALL other options, my project will die for lack of the material, is there any possible way you can help? I find that this does several things, it indicates that I havre some understanding and appreciation of his world, that I'm not some nut-case who wandred in off the street, and to some extent I make him feel needed and appreciated. I find this works wonders, and works almost every time when the person is able to help, but of course sometimes they aren't. Offer to pay for the material; and not under the table, but offer to pay his company for the material. So far I've never been asked to pay for a small bit of material. What you can't do, and still expect help, is to demand anything, or feel entitled to help. You are a beggar at his door and must act like it, but that's a small price to pay to get my hands on that piece of Unobtanium. Also, you can't tell them you are building a "boiler" or anything else that might have liability issues attached to it, and that includes a "Gas Tank." Rarely will he want to give a complete stranger the material to build a home-made boiler! Give your need another name, make it about something decorative, or anything that doesn't carry with it a potential for explosion or fire hazard. Good luck.
