One concern many people have regarding an ISO9001:2000 quality management system is if they can use outside companies to supply products or processes for their product or service. And the answer is yes, of course you can use outside companies as part of your production process. The key for passing your ISO9001 audit is to adequately control those processes or products, no matter where they come from.
If the other companies are ISO certified it does make your job a little easier, but if they’re not its still OK. Outside companies can supply product or services as part of your production process, you just have to make sure you adequately control the processes and can verify quality inspections. You’ll have to include these vendors, and all your product quality related vendors, in your vendor approval process of course.
You may want to specify in your quality manual that you conduct periodic inspections of these vendors, or that they have to be certified to a certain standard (ISO, UL, etc), or have to maintain specific kinds of equipment, or other requirements you might deem necessary to ensure their process/product meets your requirements. Its totally up to you how you do this, of course. Just keep in mind that whatever controls you specify had best be sufficient enough to pass muster with your auditor.