Seriously, 'been there, done that' fully describes my experiences with having chains for my pieces, either providing them as an 'add on' sale or including them with a purchase. There are so many different styles of links and clasps, lengths, and metals available that even if you were to have a monster supply, you probably would not have what some customers would prefer. In my opinion, we are wire jewelry artists who create and sell wired pieces, not manufactured chains and with the exceptions to follow, neither my personal students nor I sell anything we do not make ourselves (which BTW is the focus of many juried shows). The few exceptions are offering an organza ribbon or silk, suede or rayon cord (either offered with a pendant or sold separately) to provide instant gratification to the customer; to whom the versatility of a piece is mentioned, as an item can also be worn on their favorite chain, causing the piece to become more affordable because it can be worn either casually or more formal.
Of course the best (and most affordable) way to offer your work or to add a sale is to make your own necklace options such as a Finished Ribbon or Cord, or a Simple Wire Collar and sell your work 'with/on your work'.
Answer contributed by Dale "Cougar" Armstrong
Note: For manufactured chains, either finished or un-finished, WireJewelry had many different options you might want to consider. Unfinished would allow you to make the chain any length that works for your piece or your customer.