I have pulled a vacuum on my AC before - you can go about it two ways. Go to harbor freight and buy a cheap Venturi type adapter (
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=92475) if you have an air compressor to attach it to. (This is probably the less desirable of the two, but cheaper if you have a compressor - I have never done it this way though, so ask others if you go that route.)
Or, buy a standard vaccum pump - here is one that even comes with a gauge set . . .
http://sector29.com/PRODUCT_PAGES/316/316-FJCKIT6.html. This is the type of set up I use and it works quite well.
Just be sure you are familiar with how the gauges work, etc - especially when adding the R-134a. Haynes has a nice AC/Heating manual to referece if you haven't done it before - probably find it in your local. library.