Whoever sees this, congrats you found the easiest way to get this achievement (other than trading). Not many people know this, but the Rocket League item shop occasionally stocks certified items. For example, on the day I'm posting this there is currently a Striker and Playmaker certified item in the shop. This makes this achievement SO much easier. Now all you have to do it wait for an easy certified item to pop into the item shop, and you can scoop it up. Plus, they aren't even that expensive. The two from today cost only 100 credits. Good luck!
EDIT: This also applies to painted items as well they can also be found in the item shop if you are missing that achievement.