In addition to being EMS, I'm Army Guard, so, I bought a cheapo nylon/velcro trifold (In ACU) where I stick all my junk in. I don't carry a "official" wallet just for my EMS cert or FF ID.
Unless you're planning on carrying a badge (Which I don't recommend), any ol wallet will do, pick what ya like.
Unless you're planning on carrying a badge (Which I don't recommend), any ol wallet will do, pick what ya like.