。
This is what I was looking for.
Literal
I [subject-が] was-searching [nominalizer-の] [topic-は] this is [explanatory-です].
Cleft focus structure: [僕が捜していたの] [は] [これ] なんです = '[what I was looking for] [topic] [this] is.' A classic way to identify something as the object of a search. 捜す is the 'search for lost/missing' verb — distinct from 探す ('search for something one wants').