Disclaimer: I have been attending Ren Faires since 1983 both in costume and as a Mundane. If my opinions here sound a bit...sad or a one-sided...I apologize in advance.
In the Renaissance Faire world, you get what you pay for. If you are looking for good quality items, custom fit clothes, made to order items...you are going to pay $$$ for them.
My first suggestion is to go to thrift stores...yes, I said thrift stores like Salvation Army, American Veterans, various church stores. Look for dresses, tops, shirts, pants, etc. You would be surprised what you will find. I found a complete one-piece pull-over waist length 1962 leather shirt for $10! Shop around and use your imagination.
You can find patterns from McCalls and Simplicity for clothes. Of course, you can form fit clothes you make. The down side is they look rather bland. So you add items to them...pendants, maybe a baldric, leather belt to hold your drinking vessel, etc. Draw the attention away from an individual cheap clothing item and towards the rest.
The secret is to layer your clothing. Don't just wear one dress...wear 2...one shorter than the other on the lower half. For men, look in thrift stores for a XXL shirt with cuffs...you want a 'puffy' shirt look. Remove the buttons and put in eyelets and cloth cording for ties. Add a bit of leather boot flap for a bit more period look...like a pirate boot. I just have an insert that fits inside my motorcycle boots which makes them look more 'period'.
Check into the various Re-enactment groups in your area. Many do NOT require membership in order to glean information and help from the members. How do you think they get more memberships? They help those seeking information and those people get hooked! hehe
Eventually you will become more versed in what is allowed at Ren Faires these days. Many faires are no longer historically correct...they accept fantasy players too...in order to survive. Gone are the days where people would beg merchants for jobs so they could participate.
These days, it is not uncommon for a pair of Merchants Re-enactors, strolling down the avenue, talking on their cell phones or talking about the latest MMO they are playing. Kinda destroys the immersion factor for the visiting Mundanes. It is all about squeezing the strolling space down, having permanent structures, reducing hay bales for seating (keep the customers moving), and making $$$. Ah well, I am just an aging peasant pining for the old days.
Toodles,
Dolnor Numbwit
Eternal Newbie