Depends on whether you want something that looks vaguely medieval in order to go to a halloween party.
Or do you want an authentic re-enactors dress? If the latter applies, first of all you need to find your time (The middle ages were long and dresses changed) and your area (there are vast differences throughout Medieval europe, and of course other areas such as the pre-conquest midieval americans wore totally different stuff). If you want an European midieval dress, you also need to choose a social rank. As the clothes of a nobleman were of course very different than the clothes of a peasant.
Then contact re-enactors clubs and ask whether they have historical sources concerning patterns.
Authentic materials (for European mediaval) are linen and wool. Cotton, velvet, silk and the like were rarely imported via India. And only the richest of noblemen could afford them.
There are vast numbers of online sites that sell Medieval and Renaissance clothing: http://funadvice.com/r/blqlfc3oon3 http://funadvice.com/r/15c8hh6otc4 http://funadvice.com/r/15c8hh6oufv http://funadvice.com/r/blqlfc4070c ... to name a few.
The time period I am looking for is either the dark ages or renaisseance(sp)peasant or aristocrat.
you can try shopping online or try etsy.com i know they have them
Costume stores might have some?