Nail packs and paint buckets are needed to build houses in the workshop.
 
Nails and paint for building the house
 
You can collect nail packs and paint buckets by fulfilling sawmill orders. Just scroll through the list of orders to find the one that offers them as a reward. If there's no order needed, reject all orders until you get the needed one.
 
Nails for sawmill orderPaint for sawmill order
 
You can also get nails and paint by completing lumber pass missions.
 
Nails and paint for Lumber pass missions