Learn Turkish: Sponsorlar için hazırlanan plaketler nerede? - Where are the plaques prepared for the sponsors?

It comes from the verb hazırlamak (to prepare). First, we add the passive suffix -n (used after vowels) to get hazırlanmak (to be prepared). Then, we add the subject relative participle -an to make it an adjective: hazırlanan plaketler (the prepared plaques / the plaques that are prepared).

What is the difference between hazırlanan plaketler and hazırladığımız plaketler?

Hazırlanan plaketler uses the passive voice to mean "the prepared plaques" or "the plaques that were prepared", without specifying exactly who made them. If you used the object participle -dık to say hazırladığımız plaketler, it would specifically mean "the plaques that we prepared" (active voice).

Why does Sponsorlar için come at the very beginning of the sentence?

In Turkish, an entire descriptive phrase must come before the noun it modifies. The words Sponsorlar için hazırlanan (prepared for the sponsors) act together as one big adjective phrase describing the noun plaketler.

Why isn't there a helper vowel added to Sponsorlar, like there is in words like istasyon?