Well the easy answer to the headline is, "in the hamlets".

What type of troops are you building - I suspect pergs. Switch your production to Living Armour or even Gargs. To fill your queue with these takes way less soulstones (much more addy btw - that is the trade off).

If you haven't progressed your SH BG level, then A LOT of soulstones can be obtained by completing the sage quests every 10 lvls.

Other than that you need to look out for the tournament rewards.

Yep, once you've completed all the SH saga quests the only way to get a large number of soulstones is from hamlets.  You can get a few from tournaments and other rewards but not enough to keep your build queues full.  

FYI here are the numbers on amount of adamantium and soulstones each type uses up per day of building.  

Purgators: 24 soulstones 4965 adamantium

beast riders: 38 soulstones 12631 adamantium

living armor:  24 soulstones 20923 adamantium

gargoyle: 52 soulstones 34109 adamantium

These are base stats, with all possible permanent bonuses (not including the temporary 50% booster) they increase to about 3.5x.