Mix and match blocks are fun because your kids can find the "right" match or they can use their imagination to build their own pairings. Here are four examples, two you can buy, one totally handmade and one semi-handmade project for parents who want to create something for their kids without starting entirely from scratch.
1. Downloadable Art to Make Olliblocks ($5 each/PDF, Ollibird)
2. Animal Mix n Match Wooden Blocks ($16.56, Julia Staite)
3. Mix and Match Me Wooden Blocks ($12, Holgate Toys)
4. Homemade Mix n Match Blocks (MerMag)
(Images: as linked above)