A client maybe to use the ingredient suppliers of their selected co-packer or commercial kitchen to save time and money.  In most cases the client ends up finding sources for some of the ingredients (which maybe unusual or not used by their manufacturing partner).  There are a number of sources for ingredients in general, mostly being either direct manufacturers or from distributors.

From The Experts:  Will the brand take care of ingredient sourcing or let the contract manufacturer take care of it? Key points to consider before making the decision:  traceability, quality assurance, regulatory awareness, financial consideration, availability of specialty ingredients.

Notation:  Ingredient suppliers will usually give free samples to the manufacturing partner for a client’s needs to use in test runs.

Ingredient sourcing is one of the most important things to consider when questioning a co-packer. If your raw ingredients are top quality and the co-packer will do the sourcing, you must ensure they will match the integrity profile of your brand. It may take a while to find balance between the quality and cost of your product. Coming up with, or refining, your recipe to meet scale may include a lot of back and forth, including some compromise on your part. However, if you choose a co-packer that already makes the type of product you produce, they should have a list of sources and vendors readily available for you to select the ingredients you prefer.
“The co-packer usually has better buying power, which can be beneficial to the developer of the product, both in price and consistency of ingredients,” Denis notes. The exception: If you have a niche product and need a specific ingredient that the co-packer doesn’t want to source or carry, “then it is worth sourcing it out yourself,” he adds.
Before you start, run a test batch with the new ingredients so you can get feedback from customers and family, and then tweak the recipe if needed. Test for taste, shelf stability, and nutritional profile.
Source:  Specialty Food Association

PGSA’s extension teams handle ingredient sourcing for most of its clients (either for direct manufacturing or going through a co-packer).  We are partners with a number of regional and national ingredient suppliers.