My idea is a platform that connects creative/design businesses directly with small batch/indie textile/ceramic/furniture factories around the world and all orders are taken care of on the platform. I have an excel sheet of factories but how do I validate it and find the customers searching for this service
Create an MVP, that's why startups do this all the time. An MVP doesn't have to be a piece of software or technology, it doesn't have to be a functional site. Your goal is to test for market value, essentially, and find the people that would see value in this. You've already identified a target prospect in your question - small businesses that design and sell manufactured products. Ask yourself, how does your idea help them, and what sort of ROI can they expect.
There are a lot of resources online these days for getting contact information for such businesses. You can simply reach out and ask the business owner if it's something they would be interested in (and collect other useful feedback on your idea).
Otherwise I would suggest finding appropriate groups online, network locally, and basically just talk about your idea with as many people in the target market as possible. You can do all of this for little to no money, it's just your time, but this would be the simplest way I can think of to validate your idea. From there, if there is identified demand, you can look at building out an MVP and starting to get a user base you can grow.