To buy a box of cheap Luna bars, the best place to start would be supplementwarehouse.com. They are currently offering Clif Luna bars at 12 for $7.99 (Pecan Pie variety) or 12 for $11.86 (Chocolate Almond Fudge).
Bear in mind however, these boxes of bars are a few months past their sell-by date, which is why they are available so cheaply. Supplementwarehouse.com also offer a 5% discount if you find Luna bars sold anywhere else online for less. The site sells 'regular' Luna bars for $16.26 for a box of 15.
Elsewhere, allstarhealth.com are selling Luna bars in boxes of 15 for $15.29, in both Chocolate Peppermint and Caramel Nut Brownie varieties.
You can buy a box of 15 lemon zest Luna bars for $19.08 at vitaminlife.com and a similar amount of Carrot Cake Luna bars will set you back $20.44 at myremedyshop.com, although this includes free shipping.
This YouTube user found a few Luna bar-gains at her local Trader Joe's:
