It won't void the warranty, but many ppl like to keep it simple and stay with oem while under warranty. Just a easy way to be super double sure they can't question it. But in your case OEM is out of reach.
So this leaves aftermarket.
I would go to walmart and get either a fram ultra (also called extended guard if the stock isnt up to date yet), bosch distance plus, or a purolator classic. All 3 are good filters in different price ranges. Easy, fast, and most likely cheaper than an auto parts store.
While your there you can pick up your oil. (again cheaper)
Did a application lookup for the walmart filters. They list the same filter for the 2.4L, 2.0L turbo, and 2.4L hybrid on a 2012 kia optima.
Fram XG/ultra: XG9688
Bosch D+: D3312
Purolator classic: L14459