Dayacom in Taiwan have been supplying businesses for a long time and the quality is very good. They do sell retail but you are looking at MOQ (minimum order quantities) of of at least 30, 50, and up including bushings or extra tubes and that is for each plating type for each kit type. In other words if you want Gold, Chrome and Gunmetal platings you would be buying 50, 50 and 50 for a total of a 150 and the MOQ for the bushing would be another 50. You pay all costs up front and have to deal with brokerage, customs fees and taxes when they arrive which can be months after you pay.
Xiamen Rizheng Industry, also known as PenKits Mall, is in China and they make and sell cheaper versions/knock offs of most pen kits. You have to sign up to gain access to buy their pens. I don't know what there MOQs are and you will likely have the same payment, shipping and import conditions as Dayacom.
Pen kits,Turning Projects,Key Chain,Pen Blanks,Pen Boxes,Pen Case,Pen tools,Pen Mills,Drills,Supplies at PenkitsMall.com.We are an experienced and professional manufacturer of pen kits and other relevant products from Xiamen China.
www.penkits.com.cn
There are other companies in China but your guess is as good as mine as to quality etc.
I don't know what you do if there are problems, defects or you don't like the products.
I have never had the urge to make vast quantities of pens so I have never bought directly. My comments are gleaned from other pen forums over the years.
Pete