Looking for re-enacters of the Byzantine period,

[Flavivs Aetivs]

I'm not sure where you live, but you can check out HR Replikate. They sell tons of different kinds of belt buckles and brooches and militaria and other items. You can find something from the specific period you're looking for.

http://www.hr-replikate.de/englisch/home.htm

I'm not sure who in Europe sells "Byzantine" weapons and armor. SOTW here in the US has the Concentric Banded Helmet. You can try Celtic Web Merchant

http://www.celticwebmerchant.com/en/

For clothing, go with Medieval Design:

http://www.medievaldesign.com/english.asp

If they don't have it, I'm pretty sure they can make it.

A lot of what you're looking for, though, will have to be custom.