I superlove flashcards. I love the simplicity of them and the weird, crummy old paper they are usually printed on. This is a fairly new series of flat notecards, which I am continually adding to. The front includes a giant wood type letter, the first letter of an expression. The back is printed with lines for your custom message. 
ALPHABET CARDS ARE FOR SALE IN MY SHOP

Printed on 24pt manila cardstock and manila open-end (flap is on the short end) envelopes.