The maintaining or improvement of your product quality is up to you and the directions you give to your suppliers. Allowing your suppliers to supply without checking the product quality is a very costly mistake. You need to direct the manufacturers to a point that no rejects can get through the system. The reason why it is up to you is because this is a added step to the process and the manufacturers don’t want to add extra cost for themselves.
The are two approaches to take when selecting a supplier. One is the low cost and the other as you probably guessed is the quality approach. If you sent an enquiry off to a number of suppliers then you will have a wide range of costs. The area to look at is the mid to high range of prices. ordering for a supplier with lower prices might not do quality control efficiently whereas the higher cost suppliers have systems in place and more or less guarantee no defects.
The second approach os ordering from more than one supplier. The problem you are going to have is that even though your specify a material grade one of the suppliers will still try and increase their profits by selecting a lesser grade material. This means that you have suppliers suppling the same part but at different or inconsistent quality to each other.
In my experience I used to work in the satellite antenna industry in design and manufacture. We had a number of supplier supplying the same product but one supplier would always supply a lesser quality product. We found out that the grade material the supplier was using was cheaper which resulted in a dish that was slightly misshaped after forming. the material would not hold the form like the correct grade dish did. This would result in higher rejected parts and problems with picking up signal.