Cheapest place to get FRPP 373's? Also what else needs to be bought to install?

$160


http://www.blueovalindustries.com/products.php?openparent=1292&pid=

3.73 Gears
http://www.blueovalindustries.com/products.php?pid=608&openparent=1292&dealer=blueoval

26_373rgears.jpg
 
Depending on the mileage I would get a bearing rebuild kit as well. Anything over 50k wouldn't be a bad idea, which also comes with a crush sleeve, shims too. Gear oil with Ford Friction Modifier, but Royal Purple has it already inside it.

You would also need a caliper to to verify the shims and all.
I have a write up here on what is needed and the steps.
www.svt4u2try.com