I have had to source vendors several times at my work for different equipment and material needs. Sometimes all it starts with is a good old google search to get it going and find all the different suppliers. Usually you will then get in contact with a salesperson who can find the exact product you are looking for and get you an official quote for the cost and lead time. In terms managing the supplier, there may be an audit of their site to ensure they adhere to certain quality standards. You may also require a certificate of compliance with all incoming product so you know for sure that the supplier is meeting your requirements and takes the liability of any non-conformance off you. Lastly, if it is a recurring product then you will most likely need some sort of contract which dictates the terms of deliver, size of batches, and frequency.