Originally Posted by
ADSR
21 teeth is kind of a tall order. I know they make em', but you should also look into a new chainring, so you'd have more options for gearing lower or higher. Most places around where I live don't carry much over 17t cogs.
+1. Get a chainring that is in between 42-46. Then change cog accordingly. I know they are more expensive, but itll pay off in the long run. My LBS likes to stock dura-ace, and those only go up to 16. Sometimes they have other ones in, but not often