A dropshipper is a company that sells individual items and dispatches them directly to your customer – but they usually sell at lower prices than a retailer. It's challenging to define what a wholesaler is as there are such a wide variety of wholesalers doing distinct jobs, but essentially wholesalers sell goods at trade prices in large quantities.
A manufacturer is the ultimate producer of a product, the company that actually builds the TVs, cars or ovens, for example, that shops sell. It's possible to buy at trade prices from all three of these, but the most popular for small businesses are wholesalers and dropshippers, although if you can afford to buy directly from a producer then you will probably get the lowest price from them.
It can be hard to find reliable wholesalers and dropshippers, which is why services like Worldwide Brands and Salehoo exist – these are directories of suppliers. Read more about them in this Salehoo review.
Both Worldwide Brands and Salehoo are powerful tools – and if you combine them with a good online business model and a decent e-commerce platform like shopify or OS Commerce then you could have the makings of a successful business. Good luck in your business!