In a sourcing arrangement, there are typically three parties: the customer, the sourcing company, and the manufacturer. If you act as the "middleman" in such an arrangement and you would like to prevent your customer from directly contacting your manufacturer, you should enter into an Introduction Agreement with your customer. Under this agreement, your customer agrees to engage you to source the product on an exclusive basis.
You should note that, as the "middleman", in addition to the Introduction Agreement with the customer, you also need another Introduction Agreement with your manufacturer. Under the two agreements, you as the sourcing company will have different obligations. For example, in the agreement with customer, you should act reasonably to ensure that the products sourced are up to a certain standard, and under the agreement with manufacturer, you should provide necessary information to the manufacturer for the production of the relevant product.
Under the Introduction Agreement, the customer and the manufacturer will agree not to directly contact each other except with your prior written approval.