I hate to go to my string box and not have a selection of strings. With multiple guitars I am usually getting ready to put new strings on one of them.
I usually buy strings a few sets at a time. I found a place that makes buying strings online a real snap.
The site is www.stringsbymail.com. They boast $3.99 shipping for any size order. This makes them my champ for strings. They also sell some of the strings I use in singles. This gave me the chance to "fill in" where I had broken up some sets. Incidentally, they are a couple of bucks below the other dealers on the D'Addario Chromes that I use.
Most of what I like about these folks is that it is quick, efficent and no nonsense. They shipped within hours of my order and they were in my mailbox a couple days later, no fuss or muss. Very pleasant experience.