I don't technically bulk buy them, but I usually get mine at walmart, or when I'm able, sam's club. Sam's is cheaper, but at walmart I can get 10 packs for like $8.00. I'll also usually factor in 50 cents into shipping to help cover the price.
If you're really looking to buy more in bulk than that, try maybe Uline? A quick glance at their site has 100 #2 mailers (the ones I generally use for ponies) for $23, not sure what shipping is though.