A building supplier is a place where you will be able to get all your supplies, in one place. They are designed for the trade, making it cheaper for tradespeople to get the building supplies they need. If you use a building supplier, then over time, they will often give you discounts and let you in on better offers because you are a loyal customer to them. This is why it can be hard to find a new building supplier and start from scratch.
If you find yourself in this position, you should think about trying a few different suppliers. You don’t have to get everything from one supplier, so you have the freedom to explore with some different ones. Sign up for their trade memberships and over time, test out what they can do for you as a tradesperson. Over time, you will probably find you have a natural leaning towards one particular supplier.