Air lockers are sold all over the place, very easy to find. Google is your friend....
Hows your mechanical skills? You need special tools to fit them (hole tap, dial gauge, torque wrench, tooth contact dye etc). If you have ever rebuilt and installed a complete differential before then you should be ok, otherwise I would take it to someone who knows what they're doing.
Other brands of lockers include Aussie, Detroit, Eaton, Ox, Locrite, but not all are selectable.