How do you catch a sandshrew in the Safari zone?

You can use the area customizer that is located in the Safari Zone building to rearrange 12 types of area in the Safari Zone. When you do that you will be able to rotate the Desert Region into the 12 types you can visit and once you do that you can enter the Safari Zone go to the Desert Region and catch a Sandshrew.

Use the Area Customizer. If you ever talked to Baoba, the Safari Warden, and he told you to catch one, then the computer in the upper right area of the lobby just north (up) of the table he sits at should be available to use. From there, you’ll see six boxes on the bottom screen, each of which is an area. Press A and it’ll give you the options Switch, Sort, and Quit. Select Switch. The top screen will show you which area you have currently selected. The lower screen will display two pages, all of which are your options to switch the area to. There are as follows:
Plains
Meadow
Savannah
Peak
Rocky Beach
Wetland
Forest
Swamp
Marshland
Wasteland
Mountain
Desert

To catch Sandshrew, you’ll need to select the Desert option on the second page. Press A and you’ll be told all placed objects will be removed. If you’re fine with that, select yes. The area is now a Desert area, and Sandshrew are yours for the taking. According to Bulbapedia, you should have a 40% chance to encounter one. Hope that helps!