I am liking the chains on this ebay seller!I have tried different straps from other brands and sometimes it's hard to match the quality to the bag (after all, it's LV!). But I see no reason not to purchase a chain off eBay if it's the right length and comfort for you. You might have to make do a trial-and-error (take a few chance purchases) until you find the chain that's most compatible.
This seller on eBay looks like they have good shoulder strap chains but I have not purchased from them yet... but I plan to in the near future. They offer many different lengths and chain link sizes.
(incidentally, when I need hands free, I carry my regular Speedy 35 on my shoulder/crossbody with a high quality webbing strap from a Cynthia Rowley "speedy", the hardware matches perfectly)