Best way to buy a BUNCH of Battle.net cards at once?

Hello,

I run a pretty large Hearthstone club, and as prizes for tournaments we host, I like to give out Battle.net gift cards (the club funds should be used for Hearthstone only, and that's the closest I can get to enforcing how the money is used). The issue with this is that it is actually kind of difficult to buy a bunch of cards at once.

Online, I only know of Amazon, which limits me to 2 cards per transaction. This is difficult since I sometimes need to buy 20+ cards for a major event.

In store, I can go somewhere like Walmart and buy physical cards, but then I have to go through the trouble of finding a way to get the codes to everyone, especially to those playing remotely.

Is there any way I can buy a large amount of Bnet gift cards online at once for one of my events? It's rather unfortunate that I can't purchase cards as gifts through the blizzard store :(

Thanks!