You can get them at any craft store, some of them less expensive than this. For stuff I plan to use often I just draw directly on them with a marker, even a temporary marker is permanent on wood. I have a couple that I punched a 1" disc of write-on wipe-off paper out and glued to them, but I don't actually use them as much. I find a set for me marked A-E and a second marked 1-5 get much more use and I really like being able to let my players decorate their own.
While you're there, you can actually pick up 2" (2x2) and 3" (3x3) ones as well. My players have learned to fear the 3" :)
[1] Uncompensated link for example only. Not recommended, your local craft store probably has them for a fraction of that price. I usually don't pay more than $2 a bag.
From the Stampin' Place each stamp was $13.20 plus $4.54 shipping. Total $30.94.
I got the ink pad from Hobby Lobby. The brand is "Palette". It was $8.00 with tax.
I got 25 1"X1/8" wood disks on [Amazon] for $10.00 with free shipping on a total order over $35.00. I was ordering some other stuff at the time so no shipping charge on them.
For the numbers on the back I used a 8 Digit Rubber Number Stamp also from Amazon in the same order as the wooden disks. It was $9.00. I didn't need all eight digits so, I cut off the five number bands I didn't need.
All totaled $61.94