You have a few options
Sandygast makes a grand return in Pokemon Scarlet & Violet as a ghost/ground type…well…sand castle ghost thing, and this generation 7 Pokemon is super easy to find. Here’s where to find Sandygast (and its evolved version Palossand), and evolve it, should you choose to put in the work yourself.
Where to find Sandygast in Pokemon Scarlet & Violet
Sandygast is a common Pokemon that’s generally found on beaches: since it’s a literal living sand castle, after all. The easiest beach you’ll have access to is in Levincia (which is earlier in the gym narrative), to the east. You can also opt for the southeast corner of the map, or the beaches of Porto Marinada.
If you visit any of these areas you’ll have no problem finding Sandygast.
How to evolve Sandygast into Palossand
Thankfully, unlike a ton of Pokemon Scarlet & Violet ‘mons, Sandygast is a super easy one to evolve. All you need to do is level it to 42, and it’ll become a Palossand.
That’s it! Just good old fashioned leveling.
Where to find Palossand
If you have access to the northern west coast of Paldea (which you readily will have roughly halfway into the gym story), you can just go and catch a Palossand yourself, skipping the entire need to follow the evolutionary process.
This is a rarer spawn compared directly to Sandygast, and will appear on the beach above the western seaside town of Porto Marinada. Consult the map above if you need more help.