Much of the information and a number of images related to merchandise were furnished by Kevin Leahy. Thanks, Kevin!
I will add links to sources as I learn about them.
Merchandise that is not commercially available — out-of-production items, cels, and promotional items that were never actually offered for sale — can be found not only on U.S. auction sites such as eBay, but on Yahoo Japan Auctions. If you wish to bid on items offered on Yahoo Japan Auctions, bidding deputy services will deal with Japanese sellers on your behalf. Some of these services will purchase from stores or other online auction sites as well as Yahoo Japan Auctions.
Bidding services:
Shop around and compare to see which fits your needs best.