If you're running a balloon or middle weight 26" single speed set up, you'll want 10 5/8".
This is where I get my spokes from: http://www.memorylane-classics.com/
If you're doing something exotic/custom (like a two speed kick back on a double wall mtb rim), you'll want to find an online spoke length calculator. Break out the calipers and tape measure!
theres some pretty good phone apps that you put in the rim size and hub size and it calculates the spoke length. once you figure the length you need pm me i have thousands and thousands of different length spokes.im at $1.98 a spoke. just kidding. let me know.i can hook you up with some.