OEM Membership

I am considering building and selling pc’s as a side buisness. I need to find vendors for hardware and someone told me to get a Microsoft OEM membership to purchase OEM software. First where do I find good vendors? I have been searching the web but not happy with what I have found. Second, I thought you had to manufacture your own hardware or install one of your own programs to do the OEM thing. Can someone please clear this up for me? Thank so much!!