Talbot's Fine Accessories: Historical Antiquities sells actual buttons dating from the middle ages through the 17th century.
Najecki
Reproductions buttons
These buttons are primarily for 18th century reenactors, but many work for the 16th and 17th centuries as well.
Tudor Tailor buttons
beautiful, exacting reproductions of actual 16th century buttons.
Talbot's Fine
Accessories: Replica Jewellery
Reproduction buttons cast off of period originals.
The Renaissance Tailor shop
Sells hand-worked silk thread buttons
Waterbury Buttons
Sells thousands of button designs, several of which are authentic-looking for medieval & renaissance dress. Call them to see if you can get a deal on bulk purchases--some buttons sell for $15 per gross.