Here is how you can get them:
1. Just wait
Lockpicks will replenish over time. To make this process faster, increase Lock Pick Speed restoration in the upgrade menu (tap on the castle icon in your dungeon or simply swap downwards).
2. Watch ads
If you run out of lockpicks while trying to bust into a dungeon, the game will prompt you to watch an ad to get additional lock picks. Note, that you can only watch a limited number of ads, otherwise this feature would have been overpowered. They will start showing again after some time.
3. Make a flawless raid
Reach totem without a single death to replenish all lock picks. Note, that lockpicks gained by watching ads and purchased with orbs will not be restored.
You can also restore all lockpicks for orbs if you run on empty while trying to get into a dungeon.