Sounds like you’re starting your adventure into fulfillment. Most start with warehouse suppliers if they are looking to sell commoditized products (products that are already sold today). You can usually find some distributors by typing into google your industry + the words “distributor” or “wholesale”. You can also just call the top selling companies and ask them where to buy it. All work well.
Once your new value add (make sure it is) business brings in revenue, you can try going direct to each manufacturer. If you are trying to find a product to sell yourself that has not been made, the general assumption is alibaba for supply OR you can do something radical and try to find someone in the USA (you should probably do this first).