What do you call a collection of pins and buttons and cotton?

Answer 1

A collection of pins, buttons, and cotton can be referred to as a sewing kit or sewing notions. Sewing kits typically contain a variety of tools and materials such as needles, pins, buttons, thread, and other accessories used for sewing and mending garments. These items are essential for anyone engaging in sewing or textile work.

Answer 2

Many stores call sewing supplies "notions"

These are called sewing supplies and are only a very small part of a well-stocked sewing room. But it's a good start! I say, start with what you have and add to it. From you, you'll need thread, needles, a sewing machine (unless you want to sew by hand, which many people do), and a good sharp scissors ONLY used for fabric. Those are the basics and you can add to your sewing room supplies as time goes on until you are busting at the seams like I am :) I started sewing with my sewing machine on the kitchen table and a box of sewing supplies. Then, for several years I had a sewing room, and now I'm getting ready to move into a larger room in our home because my sewing supplies multiply during the night. Like rabbits. :)