You really should not need to buy it. Most of the time large companies that are doing developments puncture the piping when they are moving it around once the job is nearing completion. Once this happens, sometimes they are likely to just give it to you. It saves them money on getting it hauled off.
As far as pricing goes, its expensive. I have been told the figures by a friend that owns an landscaping company, but I can't remember off hand sorry, PM me if you still need to know.
Sometimes it can be hard to come across if your area isn't growing as far as developments go.
Home Depot will probably not carry anything big enough for what you need. As stated above though, a farm supply may.
After that, I would look for a company that does this kind of stuff and find out who their supplier is (maybe if a guy turns you down for free piping just ask him who he buys it from
We were able to get a good bit of it for free. Remember to be thankful for what you get (if you get any). We ended up offering some free rentals/admission to the owner and some of his 'high-up' people. By doing so, we set up a good relationship that may be beneficial later when they need some piping hauled of