Were is plasma found?

Plasma is found in the blood (it's the liquid portion - Red and white blood cells and platelets are the cellular portion).

And here's a bonus grammar lesson!

Where - used when asking location type question. Eg. "Where is plasma found? Where is my sock?
We're - contraction of "we are"
Were - plural past-tense of the verb "are". Eg. They are going to leave. We were going to leave.