There is a distinction to be made between the manufacturer and the supplier. The manufacturer is the company that controls the specifications and manufactures the part. The supplier is the company who sells the part to you.

In between you and the manufacturer can be a distributor, sales rep, or some other company to facilitate the sale. 

Distributors are a special class of supplier in that they have an arrangement with the manufacturer and can be counted on to have the component in stock, sell in lower minimum order quantities (MOQ), and sell product which they certify is new and from the stated manufacturer. If there is an issue, you can contact the distributor and they will sort it out. Some are better than others of course, but in general it is a trusted relationship. 

Sales representatives are like distributors in that they represent the manufacturer directly and will sell parts that are sure to be new and from that manufacturer. The parts are usually shipped directly from the manufacturer, however, as sales reps do not typically stock parts. With a Sales Rep, you are usually working directly with the manufacturer. 

It is important that you work with a supplier that can continue to support your component requirements for the life of your product. Although parts become obsolete all the time, a good supplier will be ready to help you find an alternate that is preferably a “form, fit, and function” substitute, which means that it is effectively the same part, just with a different part number or from a different manufacturer. Consider that support when evaluating suppliers. 

Consider a common electronics component supplier like Digikey, Mouser, Avnet, Arrow, TTI or another top tier distributor. They may use their own part numbers to order the component rather than the manufacturer’s part number (Digikey often appends a “-ND”), but the distributor part number can be cross-referenced to a manufacturer’s part number and a datasheet can be obtained to uniquely identify the part.  

That is not necessarily the case with the popular mechanical part supplier, McMaster Carr. McMaster does not generally reveal the manufacturer of the component, so it is hard to know exactly what you will receive. Sometimes this does not matter, but sometimes it does. McMaster does provide data sheets for many parts (e.g., hardware), so you may have sufficient control, but for other parts, it is hard to know exactly what you will get and more to the point, will you get the same part when you order more in 6 months. Technical support may also be limited without access to the manufacturer, but for many parts, maybe that is not a problem. McMaster is a good company, and fast, but be sure you are not relying on performance characteristics not shown on their data sheet. 

Now consider Amazon.com. They may be fast, but there is little way of knowing if the manufacturer will be able to supply the part continually, or if you are dealing with the manufacturer at all. Amazon is a web sales channel that might stock some parts but has no guarantee of future availability, never mind technical support. They seem much more geared towards one-off retail sales rather than scheduled deliveries. Maybe Amazon can direct you to the supplier using them for a sales channel, but it does not look like Amazon is designed to support a manufacturing effort.

Same goes for Alibaba.com. Looking at their website, they make an effort to verify the manufacturers’ name, location and other data so give them credit for trying to solve a manufacturing credibility problem Amazon does not acknowledge. Still, be sure you have found the original manufacturer and not someone who is pretending be a manufacturer while just selling parts. It is truly the Wild West (and Wild East). Both good suppliers but building a product for multiple months or years will be difficult without being in direct contact with the manufacturer. Keep in mind that good reviews do not mean good quality just like multiple anecdotes are not data.