I guess my first question is, what are you using it for? If it's for a piece of clothing that you would want to fully open and seperate (jacket, vest), then you can buy a pre-made zipper with the sliders already attached and the special tip that allows it to seperate.
If you want it for a piece of gear, then you can go with a continuous coil zipper cut to the right length. Then it's just a matter of buying the sliders you need. For shelter and sleeping bag applications you would want a double slider (pull tab on both sides of the slider) to be able to operate the zipper from both in and outside of whatever it is you are making.
Lightest and smallest? Again depends on what you want it for, but #3 seems to be the happy medium between light enough and durable enough.
As far as where to buy, if you need a seperating zipper you could check the local sewing supply store (Hancock, JoAnn, etc.). Of course, the usual suspects for mail order MYOG supply will have seperating, and continuous coil and could probably advise you further.
Check questoutfitters.com for another breakdown of the FAQ on zippers including some more answers on some of your construction questions.