Reputable sources

Do a google search on costume history and you'll find all kinds of 'information' which is suspect or even utter tripe.  Sometimes the safest thing to do is turn to period artwork for documentation, and I'd like to recommend a little place called La Couturiere Parisienne.  

For the early middle ages, I'm afraid you're out of luck here.  Their artwork collection starts with a smattering of pieces from the 1300s.  But for fans of the Renaissance and the Elizabethans, they're superb.  Their portrait collection covers many different regions and the really industrious can go on to look up the names of particular artists from their area of choice.  This is how I was first introduced to German gowns and Lucas Cranach the Elder.  

You can also visit virtual galleries like CGFA and search artists by region and time period.  There's a wealth of costume history on sites like this.