Learn Italian: Cercavo un amico che avesse un grande interesse per la natura. - I was looking for a friend who had a great interest in nature.

The subjunctive is triggered because you are looking for an indefinite, unspecific person (un amico), not a specific friend you already know. When describing a noun whose existence is uncertain, Italian uses the subjunctive. We use the imperfect subjunctive (avesse) because the main verb (cercavo) is in the past tense.

Why does the sentence use the imperfect cercavo instead of the passato prossimo ho cercato?

Cercavo describes an ongoing, continuous action in the past, translating to "I was looking for". If you used ho cercato, it would sound like a single, completed event ("I looked for").

In English we say "interest in nature", but here it is interesse per la natura. Is per always used here?

Yes, when expressing an interest in a subject, Italian uses the noun interesse followed by the preposition per. Also, remember that Italian requires the definite article for broad concepts, which is why it's per la natura rather than just per natura.

Could I put the adjective after the noun and say un interesse grande?

You could, but un grande interesse sounds much more natural. Placing common adjectives like grande before the noun gives them a figurative or abstract meaning (a "great" interest), while placing them after usually emphasizes literal, physical size.