Can all weapon mods be found?
Solution 1:
There are 5 levels for each of the 5 mods for each of the 5 weapon types. That's 125 upgrades. There are roughly 25 combat missions, so you would expect to see at least 5 upgrades per mission if you could get them in all in one play through.
Many of the wiki walk throughs list around 5 upgrades, but not all of them.
Solution 2:
Not all of them can be found laying around, some need to be bought in stores. Missed mods will show up in stores as well. It is absolutely possible to have all of your mods at rank 5 by the end of the game.