The Michigan Renaissance Festival
The Michigan RenFest is located in Mt. Holly (between Flint and Detroit on I-75). The Festival runs mid-August through the end of September, open Saturday and Sunday from 10 a.m. to 7 p.m. (Labor Day weekend the Fest is also open on Monday.)
The Grand Valley (State University) Renaissance Festival
This faire occurs on the GVSU campus, and is scheduled for October 13 & 14, 2007.
Ishpeming Art Faire & Renaissance Festival
This faire occurs in Lake Bancroft Park, Ishpeming, Michigan, typically the last weekend of July or the first weekend of August.
The Northwood Renaissance Festival
This faire was held in Interlochen, Michigan, for the first time in 2003. The Festival dates for 2004 were the last three weekends in June.
The Silver Leaf Renaissance Faire
This faire occurs in the River Oaks County Park, in Comstock, Michigan (between Kalamazoo and Galesburg on M-96). The Faire runs the last three weekends of July.
The Lake Superior Renaissance Faire
(formerly known as the Upper Great Lakes Renaissance Faire)
This faire occurs in Norrie Park, Ironwood, Michigan (in the western upper peninsula). The Faire typically occurs in June or July.
The Renstore, a Chivalry Sports Company
A good staring point for costuming and other Festival related items, including cookbooks, pewter and wooden tableware, armor and weaponry, and even tents and pavilions.
In some cases, especially in regards to the clothing patterns they offer, the items they sell represent only part of a set looking further can sometimes be very rewarding!
I've not (yet) bought anything from these folks, so I can't comment other than to say they've been around for a while now — founded in 1991, serving the online public since 1995 (draw your own conclusions *grin*).
Simplicity Costume Pattern
Having made a few Festival costumes myself, I personally feel that the best patterns from a major commercial pattern company are the ones made by Simplicity. The patterns are clear and easy to work from, and the examples shown in the illustrations are well-made (some other pattern companies examples tend to look, well, too shiny and not really all that well made).
This example (#7756 - Misses' Costume Sizes 4-20) shows just one of the many men's and women's patterns offered by Simplicity. In addition to various patterns for English or French noble garments, they also have patterns for peasant and Celtic garb.