There is no single store to get all your parts at.
Moped Lar has told you one of the best ways to find what you need.
Ebay is a good place to look.
http://www.1977mopeds.com is good if you have months to wait for delivery.
http://www.themopedjunkyard.com is good if you like to pay high shipping.
http://www.mopeddivision.com is new,so try them out at your own risk.
http://www.myronsmopeds.com has the knowledge to know what you need. (edited)