Location: Alubarna
Petitioner: Old Woman with Back Pain
Reward: Sandora Dragon Steal Tips, Herb-Grilled Bo Bo Bird, 18,000 Berries
Objective Reward: 40,000 Berries

In order to trigger this Side Story you must first free Alabasta from the clutches of Crocodile. Once Alubarna is back to normal and you've gone back to Waford you'll be sent back to Alabasta in short order. Head to Alubarna and you can find the petitioner, the Old Woman with Back Pain, in the northwest of the city's Market District. She's sitting by the side of the road, on a bench.

Shockingly, the Old Woman with Back Pain is suffering from back pain. She wonders if a visiting doctor from the Drum Kingdom could help her. She'll point you east, past the road to the palace and over to one of the two gates. From here you can go on a little excursion, talking to the people who enjoyed the doctor's help...

... or you can skip straight to the doctor himself. His name is Nafk, and he's hanging out in the Mountain Dog tavern in the south of Alubarna. You'll find it just west of the stairs leading to the huge gate that connects to the Market District. Nafk is willing to make Pain-eez for the old woman, but he lacks the proper ingredients. 

You'll need to bring Nafk two Gator Bananas for him to complete the Pain-eez. Gator Bananas are collected from Banana Gators, specifically Unripe Banana Gators. If you leave Alubarna and enter the Desert Near Alubarna you can find an Unripe Banana Gator to the east of the exit of the city, lounging in the sand near the underground path to the Baroque Works Hideout. You'll see the tip of its nose bobbing in and out of the sand.

(If you run along the west side of the street leading up to the Mountain Dog Sanji will also sniff out five Gator Bananas. They're sitting by a wagon near the back alleys in the south of Alubarna.)

Fetch two Gator Bananas and bring them back to Nafk. He'll churn out some Pain-eez with Chopper's help. Take it to the old woman - Sardan is her name - and you'll put her life back on track.