You'll be introduced to Demiguise Statues during main story quest "The Cartaker's Lunar Lament" for Caretaker Gladwin Moon. He'll also teach you Alohomora (unlock spell) as most of the statues are behind locked doors. You can reach 23/30 of the statues with Alohomora I, however the remaining 7 require you to have Alohomora II. To unlock Alohomora II just head back to Gladwin once you've found 9 or more statues and he'll upgrade it for you. Handing in a further 13 (22 in total) statues to Gladwin will unlock Alohomora III for level 3 locks.
Note: The Demiguise statues can only be collected at night. Just open the world map and click "Wait" to transition into day/night.
For the locations for all 30 Demigusie Statues please see my video guide below: