NEVER replace single strings!
As you play your strings aquire dirt and grease, as well as stress points and wear. This all dds up to having an uneven tone accorss the string (dead spots) and general lack of tone. By replacing a single string you will have a really uneven sound, one string being amazing and rest uneven and crumby.
Just bite the bullet and buy a new set of strings. I strongly recommend D'addario strings. I have never had one break, they truly last (I really mean it! EB and those awful rotosounds just die so fast in comparison), and sound better than any other string I have ever played (even Elixors).
Another thing to sort out is why the string even broke. Check your saddle for rough edges, and your nut for a smooth, good fit that isn't too tight. If you need help see a guitar tech, doing so will improve reliability, tone, and playable response.
