Just download Apache (www.apache.org). It comes with it (in ApacheX.X/bin)
...Guessing (from one of your other questions) that you're using a Mac... there appear to be instructions here:
(if not, I can probably help with a Windows installation, but in general, Google is your friend!)
A list of mirrors for the windows binaries can be found here.
There are some basic instructions here:
...basically, install it, and the ab.exe will be in the 'bin' subdirectory of the installation