I created my own also but for the 16 channel version.I tried the cards from G&G. They work OK, but not the best. Might need some trimming to fit. Not to mention the cost (oh wait, someone did mention it...)
I finally made my own cards by scanning an original card and cleaning up the scanned image. I then created the attached pdf file.
To make your own cards, print out the attached file on heavy cardstock (standard letter size, 8.5 x 11 inches), then use a paper trimmer to cut out each individual card. The original cards are 4.724 in x 2.224 in (12 cm x 5.65 cm).
When printing, be sure to set the print scale to "actual size" (not "fit" or "shrink", both of which will print it at 97% scale, making the cards too small). Also, use a black marker to color the entire back side of the numbered area to make sure that no light leaks through.
If you follow the above instructions, the cards should work fine. They do for me.