Dryads + Blights drop Heartwood, and it's not a very high drop rate. More reliable ways of getting it will show up soon - but the cost for the upgrades will likely get balanced as well making it less of an issue. If it helps, none of Scout's interactions are locked behind upgrades. They are strictly there to improve you brewing. Scout's interactions are unlocked using the Study Desk and (initially) researching the Undead.Really enjoying this game. My only issue is conversations are 1 time things and I can't for the life of me find heartwood.
(edit: Think I got one from fighting the tree monster that uses magic but man that is super rare / slow if you need 10)