Mods are acquired by dismantling an item with a mod. Not sure if there are other ways.
Hovering over the dismantle option will show the resources and mods obtained from dismantling marked items. This helps deciding to sell or dismantle.
Edit:
@Good mentions another way to locate the mods you need in the comments:
"If it has the little square looking thing beside the mod means you already have the mod so just sell that one!"
Edit 2:
The little square thing is also visible when selecting a reward so pay attention to it to get an item with a mod you need.