You can buy expensive ball pool balls from a toy store and pay a fortune, or you can improvise like I did.
I bought a bag of water balloons and they cost me next to nothing.
I filled less than 100 balloons with air by using a bicycle pump and put them in a box as a ball pool. Then I sprinkled some kibble in there and asked Miki to search.
Miki loved these balloons very much and he wanted one of those balloons to play with, when I started to fill them.
Later on we got a chance to try this with real ball pool balls. Miki seemed to love them as much as the balloons.
You can alter the game is you replace some of the balls with hollow laundry balls. Just remember to clean them properly before you them to your dog.