You might want to try Benton & Johnson here in the UK. Years ago Soper Lane had gold thread made by them, over a silk core, yes, it was pricey, but they are really helpful and I don't think the minimum order was actually too large at the time.
Would I order/use? Well, I have, and do. Problem is, I do have quite alot still! Though, one day it will run out and more would be needed... What I've found is that because it is so special, the end product (after all other costs and time and work go into it), tends to be very pricey. So, fine for something which I will 'showcase', but out of the price market for most people. (Most of my orders still use a silver gilt thread, but with a cotton core, as this is 'standard', so less pricey) Saying that, there's always someone who will come along and surprise you!
Oh, and laying down two threads when brocading is quite normal in medieval TW too.
Occasionally other cores are used historically for these threads. Nancy Spies' book, Ecclesiastic Pomp and Aristocratic Ceremony, analysises the types of metallic threads used in tablet woven brocades and may be a good read for you to be able to check other options too.
I'm watching your whole project with great interest and can't wait till it's up and running by the way!
I, on the other hand, probably would not buy genuine medieval thread, even if I ever do develop the nerve to attempt genuine, Viking-style brocaded weaving (which is the only thing I would be likely to want it for). But then, that might well depend upon how "pricey" it ended up being.
Good luck with your project!
Have you thought of ordering with Tricia of Thistle Threads? It appears she has also had someone develop a real gold thread in the recent past (for the Plymouth Jacket project):
http://thistle-threads.com.mytempweb.com/blog/index.php/2008/07/theres-gold-in-them-hills/
http://thistle-threads.com.mytempweb.com/blog/index.php/2009/06/real-gold-and-silver/
http://thistle-threads.com.mytempweb.com/blog/index.php/2009/05/threads-stripped/
http://thistle-threads.com.mytempweb.com/blog/index.php/2009/05/threads-wrapped/
Thank you all for your hints and helpful comments; I'll look further into things.
Gina, thanks for mentioning the use of two threads at once in weaving, too - that had slipped my keyboard, I was so concentrated on embroidery at the time...
I know that there are other cores used, but most common in what I have found is silk. And I've already got an easy one, because the silk is naturally coloured, not dyed red or yellow.
My hope is that I can offer the thread for an acceptable price; for that, I will also consider investing in a whole kilo. I need to think about it first, though, to be sure about my options and to have an idea about how much the thread would cost in sale. You'll read how it turns out in any case.