Fat Cat Achievement in Little Kitty, Big City

While the game and this achievement seem to imply that Shinies are a finite collectible that you must hunt down around the city, that is not true. Shinies come from many places:
 

• Located randomly around the city, especially on climbable areas and platforms
• Underneath orange road cones
• Inside some random bushes, barely visible
• From recycling cans
• Inside bird nests
• Inside trash cans
• Inside glowing breakable jars

You will assuredly come across plenty of Shinies while doing all quests. Personally, I didn't need to grind any, but there is actually a simple and easy way to grind infinite Shinies from recycling. Basically, go to any red vending machine and run at it with  to headbutt it (the first time you do this will unlock Who Needs Cash? (25G)). This will knock out cans. Typically, there is a blue recycling bin next to all vending machines, so you can then pick up those cans and deposit them in the recycling bin. Each will give you a Shiny as a reward. Then, pause your game and save it, then immediately load your save. The vending machine will restock, allowing you to repeat this process of recycling cans. Pretty much every vending machines gives two cans, but there is a red vending machine between grandma's market at the playground that gives eight cans. This is the best place to grind out Shinies (and Litter Picker (25G)).