Slurp barrels can be found in the fenced-off backyards of gas stations or on a ship that spawns in a different random offshore location on each map. Gas stations are good drop spots because they have an abundance of loot, chests, vehicles, and vending machines. Some of them even have ice machines. They usually don’t have that many Slurp barrels though (typically five) and can be dangerous landing spots due to their popularity.
The ship typically carries twelve Slurp barrels, but given that it always spawns at sea, it’s going to put you at the edge of the map. This can be a disadvantage in terms of avoiding the storm and makes it tougher to find loot other than Slurp barrels.
So, there are plenty of Slurp barrels out there, but each one only contains a small amount of Slurp. If you really want to restore a lot of health and shields in one go, then you’re better off breaking open a Slurp truck. There’s one of these half-buried in the snow at Frosty Fields, which is what Tilted Towers is called now that it’s frozen in snow and ice. Smash open that truck and you’re pretty much guaranteed to max out both your health and shields.