When I've changed mine (yes more than once for various reasons) I've used Ford's O2 sensors. They are about $40 USD each.
Install is very easy. Just get yourself a 22mm open-ended wrench (I think it's 22mm) and take off the old ones. Then put a little bit of anti-seize compound on the threads of the new O2 sensors before screwing in the new ones.
Make sure not to get any anti-seize compound on the sensor tip.
Install should take about 15 mins.
Good luck.
E
------------------
1991 5.0 LX Coupe -37,800 miles
13.17 @ 106.14 mph w/ 2.138 60'
Pro-Charger D-1SC on the way!!!