Shop carefully; if you have done any sewing or worked with fabric, you should have a general idea of how much it costs to make a garment. (If you don't have this experience, ask a knowledgable friend - or us! - if what you've picked out is reasonable.) Some festival merchants, I'm sorry to say, have little compunction about driving up their prices because people perceive their merchandise as a novelty, and don't think about the prices as comparable to everyday clothing. I personally have had some experience with Chivalry Sports, and have found them to be reasonable and offering quality clothing, although sometimes slow in shipping. Moresca is everywhere on the festival circuit, and has fierce adherents and fierce detractors. I think their simpler clothing is much nicer, and devoid of some of the ill-advised trimmings of their more complex garb; I do think, however, that they are overpriced.
If nothing else, though, these all make for lovely window-shopping!
Chivalry Sports - This site incorporates the full-fledged magazine style of their catalog, with articles and information surrounding the merchandise proper. Lots of pretty things.
Moresca - The quality of the photographs is not terrific, although you can order their catalog. Consider their prices carefully (see above).
House of Dra Costumes - Don't know much about them, although some of their pictures look nice. (The site does have a sound file that loads automatically. Ick.)
A Wardrobe In Time - Again, I admit that I chose it for the pictures. That, and the castle metaphor.
Merlyn Custom Costuming - More pretty pictures.
Howdy To Your Face! - More pretty pictures. And their web site is very cheerful. Good prices.
Argent Flame Costuming - Reasonably large variety, plus a nice gallery of period images for ideas.
Renaissance Dancewear carries a number of different things, but is mostly notable for their extensive collection of tights. (Well-fitting opaque tights are essential for some men's styles!)
SCA Merchants' Row isn't just clothing, but all manner of items for the historical re-enactor. Look at the clothing merchants, certainly, but be sure to browse about and see what else there is to see.
Black Swan Designs - Meticulously made clothing; a bit pricier, but I'm more comfortable with some of their prices than, say, with Moresca, because the cost reflects time spent.