I was looking for copper to build a counterflow chiller and I found a good price at Ace Hardware.
Lowe's had it for something like $77 for 20 feet or something ridiculous. Ace had 20 feet for $25. You can find it on their web site. I got mine at the local Ace. I wanted 25 feet but the precut package only came in 10 or 20. The guy said he could cut me 25 feet from a big roll in the back but that was almost $3 a foot! He said it was the same copper but he didn't know why it was so much more expensive.
There is also 25 feet of copper on eBay for about $25 (search for copper tubing) including shipping. But it says it is .020 wall thickness, compared to the .030 that you usually find. I'm not sure if this makes a difference or not.
Just thought I'd let you all know what I found in my recent search for copper.
Oh, and that's for 3/8 OD soft copper.