Okay...here's the deal...I've got a Shimano Ultegra cassette that is a 14-25 (14, 15, 16, 17, 18, 19, 21, 23, 25). I'd like to have a cassette that includes a 13. Ideally, I'd like the 25, but I could live with a 23. Is there an easy way to convert my 14-25 to a 13-25 or 13-23? What size sprockets would it include?

Another question to go along with this; I'm assuming sprockets can be bought individually, so does anyone know where to buy these? Where would I need spacers inbetween the sprockets for a 13-25/13-23 (I also don't know why they're there, if someone would like to explain that to me while we're at it!).

Thanks!