Pokémon in the underwater Bubbly Basin area of Pokémon Pokopia have different comfort needs than anything you dealt with on the surface, and one of the first requests you'll run into is a Pokémon asking you to make their home warmer. The solution is simpler than you might expect, but finding the right item does require knowing where to look.
What is the best way to warm up a Bubbly Basin habitat?
A 
Heat Rock is the item you need. It radiates heat to the surrounding area, making it the go-to fix for any Pokémon complaining about a cold habitat in Bubbly Basin. Place it in the habitat and the temperature issue resolves.
The Heat Rock belongs to a set of four special rocks in Pokémon Pokopia, each designed to modify the local environment in different ways.
All four are rare items, so you will not stumble across them constantly. The Heat Rock is the one to prioritize for warming up any Bubbly Basin habitat.
Heat Rock directly inside the habitat that needs warming rather than leaving it somewhere else in Bubbly Basin.
Bubbly Basin cave entranceWhere to find a in Bubbly Basin
The most straightforward way to get a Heat Rock is to grab the one sitting inside a Pokéball in a cave within Bubbly Basin. From the gate entrance, head into the main area where 
Popplio first guides you. Look for a cave on the right side of the stairs — it has black 
stone above the entrance and scaffolding pieces at the front. Head toward the back of that cave and you will find a Pokéball containing a Heat Rock.
That is the one guaranteed spawn, so if you have not collected it yet, that is your fastest route.
Heat Rock Pokéball locationWhat are the other ways to get a ?
Once you have found your first Heat Rock, there are a couple of ways to get more.
- 3D
Printer: Take a photo of the Heat Rock you already own, then use the 3D Printer to create a duplicate. This costs resources, including Poké Ingots, which are not cheap, so plan accordingly. - Cloud Islands: If you have not found the cave Heat Rock yet, some Cloud Islands carry one. Traveling to a Cloud Island that has one lets you
collect it without needing the cave first. 
Gimmighoul and 
Gholdengo trades: Trading with these two Pokémon can yield a Heat Rock as a reward. The item pool for these trades is broad, though, so this method is not reliable as your primary approach.
Printer requires Poké Ingots, which are a costly resource. Save duplication for items you genuinely need multiples of rather than burning Ingots early.For players managing habitat comfort across multiple Pokémon, knowing how to increase Comfort Level more broadly will help you stay ahead of requests like this one.
3D Printer duplication menuHow does the fit into Bubbly Basin habitat management?
Bubbly Basin sits underwater, which changes the rules for several comfort mechanics that work fine on the surface. Temperature control through the Heat Rock is one of the adjustments the DLC introduces specifically for this environment.
If you are also dealing with humidity-related requests from your Pokémon, the approach there is different. Check out the guide on how to increase humidity in Pokémon Pokopia to handle those separately.
For a full picture of every Pokémon available in the expansion, the complete Bubbly Basin DLC Pokémon list covers every confirmed addition and what to expect from Parts 2 and 3.
For everything else the Bubbly Basin expansion adds, the full Pokémon Pokopia guide collection has you covered across habitats, Pokémon spawns, and comfort mechanics.