When buying spokes..try to purchase the correct gauge spokes to correctly fit into the size holes in your hubs.They are selling lot's of generic low quality spokes on Ebay that aren't the correct gauge size and you'll end up breaking them and having to respoke your entire wheel over again.I like the O.E. spokes best because they are butted sizes that fit well in the hubs.There are other spoke manufacturers...;when you do get some of them, specify the correct gauge size holes in your hub to match the spokes you order from them.I wouldn't get any that are smaller than the O.E. gauge size for safety reasons.
To answer your last question,folks always start from the beginning to respoke the entire wheel w/ fresh spokes..,if you do one at a time you would have to bend each spoke which would weaken it a little.