Cheaper than what? DJM is only like $500 new for the parts, plus shocks. Making a C notch isn't hard if you have a welder, but that's really the only place you could save a few bucks and if you're asking the question, I imagine you don't have a welder.
Every once in a while, used stuff pops up but it may only save $50 or $100 by the time you are done, or something is missing/not usable from the used kit and you end up spending money to make it work anyway. There are some really cheap kits on ebay...but at some point safety becomes a concern, as some of the parts I've seen are pretty obviously subpar. Generally better off in the long run saving a bit longer to buy something with known quality and support.
Don't forget the shocks when budgeting for this. Probably the most important part of making it actually drive and ride okay/safely, and the main reason people complain after dropping their truck. Definitely an area where you get what you pay for, but sometimes you can find used high quality shocks in the right size from the muscle car and circle track world.