Instead of paying for store bought flash cards,with just a couple sheets of poster board and paints you can design your own high contrast flash cards. These cards are great for early stages of infancy when they cannot see colors and darker shades are easier to differentiate. It isn't until around 6-8 weeks that a baby will begin to focus on items (although you can start using them at birth), so you can feel free to make only a handful (your time for crafts, like mine, may be limited) and then expand as baby ages.
Here are 6 flash cards I painted on black and white poster board cut-outs:
Later I may add upper and lower case lettering as I plan to create a card for each letter of the alphabet.