I bought some monster and hero packs and in the process of playing a campaign discovered I've lost the quest pages out of two of them. Is there someplace I can download the quests or am I gonna be stuck buying the kits again?

Campaigns and Quest leaflets are one of the things I really wish FFG would make available via PDF.

The tiles and models are more than enough to justify purchase of a given expansion, so having the Quest Encounter details 'in box only' just leads to silly situations like this, and makes sharing encounter rules with your group that much harder as you cant circulate the PDF ahead of time.

You can try to contact FFG to get it replaced, but to my knowledge there is no online resource available.

There is no legitimate (read: legal) source for what you lost online, as the quest guides are copyrighted. I would encourage you to contact customer support and explain your situation. It is likely they will be able to send you replacements.