Click on "Fasting Center" and read all the info on those pages before taking on a juice fast. It gives a pretty good overview of the concerns, benefits, and proper ways to go about fasting. I've juice fasted 3 times for a one week period each and it was wonderful. I think the next time I'll shoot for 2 weeks or so.
To directly answer one of your questions: Yes, you SHOULD NOT use bottled juice from the store when juice fasting. These juices are cooked (pasturized) for safety reasons, but this pasturization process diminishes many of the vitamins and minerals that are helpful to your body during a fast. Also, with the exception of orange and apple juice, most juices from the store have Sugar and other additives in there and these are definitly to be avoided while cleansing with juices. I have a juicer at home (I paid about $65 for it on E-bay and I love it. The brand name of mine is Waring.) Let me know if there's any other info or advise that you'd like. And if you decide to go for it, just know that the first couple days can be hard, but after that, the feeling of juicing is amazing!! Best wishes!