You can either get this over time by playing the game and collecting gems, or you can take advantage of the Gem Jug dance to get infinite money after the first run of Mermaid Falls to max all your relics and stats.
The snake lady is at the end of the second part of Mermaid falls, just after all the ring climbs, and just before the slide. Buy the Warp Dance from her and trade for the Gem Jug dance.
After finishing the level, you can go back to the bath house in Scuttle town and transform into the Gem Jug in the water to get infinite magic and keep hitting buttons to get infinite money.
Make sure to buy the magnets first so it's easier to collect the money.
You buy relics and upgrades at the shop in Scuttle town.
The walkthrough by PistolMcAwesome
does a nice job of laying this out for you: Shantae: Half-Genie Hero Walkthrough
(Page 5)Shantae: Half-Genie Hero Walkthrough