Holly put that photo up on the facebook page she created for Just Add Light and Stir.
The blocks come apart by removing a couple of screws. They're plexiglass in wooden frames, and came from Discount School Supply.
Refillable Window Blocks And at the bottom of the top photo some of these show—they have one open side so they can be nested.
Nest & Stack with Color Blocks - Set of 6
I got them when they were on sale. I like that company. They deliver quickly and are honest and generous. I get paper there to wrap books in to sell, and have gotten gifts sometimes, and the tunnel kids play in at the Always Learning Live gatherings in Albuquerque.