I'm a big fan of these guys.http://www.medieval-market.com/
They pretty clearly label the centuries you'll find stuff in (check with your authenticity officer if you're unsure about anything or want a clearer date) which a lot of traders don't do because they want to make a sale even if they're giving you inaccurate crap.
I get my linen stuff from them - usually the 'average' stuff, though I notice it's a bit thin - possibly thicker for their top grade stuff? Which is preferable, especially for braies.
Can't comment on their wool stuff - haven't ordered anything because I'm unsure about what sort of colours their dyes are. Some are more accurate than others.
Their gambesons seem pretty cool as well, I'm trying to see if I can get one that's knee length whilst still being fastenable at the front - which is farby as fuck, but once you've filled a gambeson with sweat and have to get someone to help you out of it, you'll appreciate having one which goes on like a coat.
I have an arming cap in natural linen by them and it looks great, only criticism is it's thick as hell, I should probably have ordered it with two, or maybe only one, layer of wadding, instead of the default three.
You might have to search hard to find some of the 'early' stuff they have, I'm sure I've seen some, even if it's just cool little accessories. They definitely focus on the high and late periods.