Both types are available in fixed lengths, and in a continuous roll. Chances are pretty good that you won't be using a continous tooth zipper, so we'll leave that one out of this discussion.
Coil zippers have little plastic coils that are stitched onto the zipper tape.
They are typically used on packs, tents,and lightweight clothing. YKK is the standard brand used in almost everything. You can find other brands at the local fabric store, but in my experience they just aren't as strong. All of the sources listed on the Tips page carry an excellent selection of YKK zippers. When I put a zipper on, I typically set the needle at the far left position, in order to be as close to the zipper teeth as possible when stitching.