I could write a book on this- spent two days doing it without any special tools. Get a large spark plug socket to sit around the stem and collets and hammer it on the retainer (preferably with a cloth around the socket to stop flying collets). If the collets don't pop out you aren't hitting it hard enough, short sharp blow. Make sure the head is positioned so valve itself wont hit your bench while moving or you could bend a valve. After a little practice you should be able to do it with only one or two whacks of the hammer. Do it in a clean environment so you don't lose any collets, I doubt anyone will sell you one by itself. The order of the valves and collets in their original place is not important, you can mix and match (but not mix exhaust or inlets valves obviously)

Re-Installation is a little more difficult and requires a lot of patience, I will detail this after work if you want.