So, my Home depot only carries 20 ft rolls of 3/8" copper tubing, but I'd like to get 50'. Has anyone purchased this stuff on-lne? If so, where do they have it?
Find another place locally. There's no such thing as deals on copper, it's a commodity product. Most places are going to sell it for roughly the same price. The shipping will kill the price. Someplace around you must have it.
I'm not sure of what kind of places you have around there, but I used to work at a metal recycling company and I'd come across 30-60ft spools of brand new copper tubing all the time that wasn't selling and companies were just trying to unload it for the cost of the copper. Might be worth it to just try calling some recycling companies around you and ask if they have anything.