For those wondering where these cards came from, they were included with the anime's OP single CD (Rei and Saeko) and the ED collection CD (Saya and Shizuka). I also own the HOTD general soundtrack and it didn't come with any cards like this, so I think these four are all there are.

Unfortunately Saya's and Shizuka's cards aren't nearly as crisp-looking as Rei's and Saeko's, but oh well.