The most common type of hidden object game is the murder mystery. Famous writers like James Patterson have games loosely based on their works out, as does the TV series CSI: Las Vegas, and there is a large market for similar games, such as archaeological searches and other investigatory types.
In terms of finding these games there are many options. Google itself has a Shopping section, which aggregates all the biggest name vendors as well as eBay sellers and other smaller businesses selling this game, and provides the option to select by rating, price, location or company.
Aside from that, well established websites like Amazon have specific search categories for different consoles. You could easily search for the Nintendo DS and find the style of game you're after, as well as search keywords.
In the UK sites like Game, Gamestation or Play also have specific search categories too, and for US customers you'd look somewhere like EB Games.
Hidden object games shouldn't be confused with an Easter egg, by which I mean a secretly embedded extra bit of game or footage. These are in some DVDs but not DS games as they do not have the capacity.