sales@goldenmotor.com is the official source for buying from the manufacturer. Purchases through sales@goldenmotor.com are at the stated price on the DIY e-bike kit page from their website. They have a distributor in northern California, who receives the kits in bulk and he forwards them on to buyers. They are supposed to be QC'd at the factory, but users have gotten kits in various states of damage after shipping...something I think goldenmotor intends to remedy soon. If user testimony from the boards is any evidence, goldenmotor has been plagued by slow response times, occasionally not responding at all to e-mail inquiries, but generally responding to paypal payments. I have not yet heard of a user who paid via the web site and failed to receive a kit, however shipping times can vary greatly. Shipping times vary based on whether there is current inventory in the US, or if they have to wait for the next container shipment to arrive.
ebike@verizon.net is another avenue for buying a motor kit. 'Myelectricbike' (the user behind the e-mail address) buys kits in bulk and tests them prior to sending them on to users. To the best of my understanding, he is selling the kit at the website list price with the same set of accessories as listed on the web site. myelectricbike will let you know promptly if he is out of stock.
www.ampedbikes.com is run by user 'dray' on these boards. He buys kits from goldenmotor in bulk, provides some additional QC, adds a few additional accessories beyond those listed on the goldenmotor site (battery bag, bike rack, etc.) and sends them on to folks who buy through his site with a mark-up for his troubles. Prices and additional accessories are listed on his website. I'm pretty sure he currently has stock on hand ready to ship.
-Mike
[edit... noticed my ' ' markings for usernames was inconsistent)