Last week I had a patron and her son ask where the I Spy books were, so I showed them these books:

...which were the ones they were thinking of, so that's good. However, I also showed them a few other series they didn't know about, and they decided to take the new ones instead. So here's a quick list of some other search-and-find types of books in case your patrons need some fresh ideas, too.
Usborne "1001 Things to Spot" booksShelved by author; a keyword search for "Usborne and spot" will find them all.
Each two-page spread has, around the perimeter, images of a number of particular things to look for, like "6 sharks" or "10 magic wands."
Spot 7Under E Kidslabel.
In these books, you have to compare two pictures and spot 7 differences.
Look-AlikesUnder E Steiner.
The author uses everyday objects like paperclips, dollar bills, and crackers to create her pictures.
Where's Waldo?Under E Handford.
The Grandaddy of Them All! Check your shelves; you may have gotten fresh copies in.
Can You See What I See?Under E Wick.
An "I Spy" for the younger crowd.
Let's Find Pokemon!Under E Aihara or E Pokemon; a title search for "Let's Find Pokemon" will get them all.

Walter Wick and Jean Marzollo have some
I Spy books in the Easy Reader section, too. They are under ER Marzollo or ER Wick, a keyword search for "I spy and easy reader" will find them.
For younger kids who want to search, but find these other books a little hard, here are some titles that have worked out as I Spy books:
Faraway Farm, E Whybrow

Alphabet House, E Wallace

My Very First Word Book, E 428.1 WILKES

Last but not least, if you need to find this type of stuff, try a word/keyword search for "picture puzzles and juvenile."