There are always plenty of 9x12 plate holders on Ebay, mostly from Germany, it's worth logging into Ebay.de (same Login details work across all Ebay sites, US, UK, Germany, Italy etc) but I've also bought from Slovenia, usually the best are in sets of three with a wallet. The problem is sellers aren't always certain of what they are selling and so unfortunately you have to buy without being totally sure of the exact type and fit.
Re-furbishing the holders is quite easy, usually a quick rub with some wet & dry removes all the rust & loose paint and a quick spray with black or dark grey primer then satin black paint. The velvet light seals are also quite easy to replace. In fact the first holder I restored was the one in by far the worst condition, jammed shut like Nathan's and quite rusty, and it's fine now.
I wish there was an easier way to buy the right ones but I've not found it yet.
Ian