The Digger Drill has but one purpose: to let you create (or move!) your Secret Base. The Underground Man gives one to you for free, but you’ll have to purchase one if you want to move bases. The Underground Man doesn’t sell them, so now you may be wondering who does.
The Digger Drill is sold by the Hiker in the Grand Underground that trades items for spheres. There’s always one Hiker Trader in every separate section of the Underground (such as the underneath of Jubilife/Oreburgh and Snowpoint City).
The Digger Drill won’t always be sold since his wares are randomized. Fortunately, his items reset every midnight at your local time, so you can check once per day. Each Hiker of an Underground’s sector also has a different set of items up for grabs. In short, there are multiple chances to check which Hiker has a Digger Drill and a fairly high chance that you can purchase one every day.
Related: What to do with Digletts in the Grand Underground in Pokémon Brilliant Diamond and Shining Pearl
If you need a place to start, the nearest hikers are those when you go under in Oreburgh City (go right and up) and Pastoria City (go up).
The Hiker also offers pedestals and TMs. The TMs are also randomized, but there’s always a chance for all 100 of them to appear at some point.
For more help with Pokémon Brilliant Diamond and Shining Pearl, we at Pro Game Guides have you covered.