Where would you look for some "original" planetesimals left over from the formation of our solar system?

Asteroid Belt and Kuiper belt

Explanation:

When solar system was forming a the leftover material was concentrated in Asteroid belt (between Mars and Jupiter) and Kuiper belt (which starts just after the orbit of Neptune). These two belts contain various small bodies called as planetesimals or asteroids which can give a lot of information about the early solar system.