Basically you need something to cover 9.4 inches in height and 6.6 inches wide. So you could look for the iPad Air 2 and current iPad 2017 for one.
I paid $40 for my 4th Gen Apple Smart Cover during Apple's Black Friday sale a few years ago. Its now starting to wear at the hinges but not too noticeable. The Apple Air 2/Apple 2017 smart cover sells for $39 . The Apple Brand is the only one I can find. If you check local stores like Target or Marshalls you may for them, you might find it for less. The other options have the back covers attached.
I'm looking for one myself to added to my 10.5 iPad when I get it. I like that setup. Black Friday is coming up in about a month. Maybe there will be a deal on one? MacMall B&H Photo Amazon Adorama Best Buy.