Hammers are needed to smash Crazy Eggs (60-70 needed to activate all 4 eggs per cycle, up to 100 if unlucky); normal monsters otherwise don't drop hammers, making them hard to farm at scale. Ranked sources: hunting bosses with a Double-Drop general (e.g. Bivals + King's Ring + Hurricane Bird, ~55% double-drop) gives roughly a 50% hammer drop rate regardless of boss level - a survey found 44 drops from 85 kills (51%) across Lv1-8 bosses (Zombie through Peryton, 2-7 hammers per 10 kills each) - so it's faster to farm low-level bosses at the map edge than to chase high-level ones; Vikings drop hammers less often (~28%, 14 hammers from 50 Easy Vikings). Gathering resource tiles Lv6+ also yields hammers, roughly proportional to tile level - a Lv6 tile gives about a 70% chance per 1-2 hour gather (higher-level tiles have better odds but take 5-12 hours and are harder to find repeatedly). Raise gather speed via a high-Political-stat general, a gathering-speed skill general (e.g. Lucy), Monarch Gear crystals equipped before arrival, and the 'increase all resource gathering speed' city buff item.
Original research source: Evony Guide Wiki