To me, and this is my personal opinion, so feel free to chide, what takes a garment from a costume to a reproduction is the combination of correct construction and correct fabric. Using only one of these doesn't make an inferior garment, but it does make a garment that isn't a true reproduction. So, when reproducing garments of the past, we have to be aware not only of the look and general shape, but of the construction techniques and the fabrics used during the period.
Ironically, while construction technique and pattern feels like it should be the trickier of the two, once one has the right sources (and they abound for the eighteenth and nineteenth centuries) it's easy enough to decipher. There are even pattern companies that cater to reenactors (though this can get tricky--the unfamiliar should seek the guidance of someone more experienced, as many commericial patterns aren't quite right). But fabric--commonplace, ordinary fabric--can be a challenge.
The biggest sticking point for us eighteenth-century people is that the most commonly used fabrics by ordinary people--linen and wool--are tough to find and rather more expensive than our 21st century staple, cotton. Cotton was used and available in the eighteenth century, but was pricier (remember, no cotton gin) and usually of weaves somewhat different from our modern, standard "broadcloth" and, when printed, in particular prints not often available today. Oh, prints. Prints, you will be the death of me. Many a lovely reenactor's clothing has been marred by an incorrect print. And--this is my super-anal pet peeve--linen was usually used for undergarments like shirts and shifts. The look of a cotton shift or shirt just has that almost but not quite right look to me. Super-anal pet peeve moment over, and my apologies.
Thank goodness for the internet--before its advent, we were consigned to shopping at whatever fabric stores were local, and while I love JoAnn's, and my best-ever fabric score came from Hobby Lobby, the selection is often limited, as their primary audience is crafters, costumers, and folks creating modern clothing. Go figure, reenactors don't top the priority list. But now--now the niche retailers that have been selling to reenactors for years have a wider market base online, so anyone can order from them (though I recommend the experience of visiting them in person!).
Wm. Booth, Draper : In the interest of full disclosure, these folks are friends of mine. They are awesome people--very willing to talk historical clothing and very willing to teach. And their fabrics are divine, varied, and all more than vetted historically-speaking. They also carry a well-edited selection of patterns
Burnley and Trowbridge: Also a (tent) shop I've had the pleasure of visiting in person. Again, great selection, edited with the reenactor in mind.
96 District Storehouse: The location of this weekend's fabric shopping. So much wool. So much linen. A small selection of lovely prints, and marcella fabric to boot. Plus the largest selection of silk ribbon I've ever seen. Their website doesn't have much on it at the moment, but I imagine you can call for more availability, or try to catch them in person. Worth it.
The following aren't reenactor shops, but they are good resources:
Fabrics-store.com: Sounds generic, but they have a wonderful selection of linen in a wide array of colors and weights. I've never been disappointed (the bolt of linen came from them).
Denver Fabrics: This used to be my go-to for silk, but their selection has been more limited lately. They do still carry a decent dupioni selection, but be wary of the dupioni--when it's super-slubby, uber-textured stuff, it's not the best for repro fabric. Some dupionis are great as substitutes for the more elusive silk tafetta, but some are just too over-the-top in their crinkly slubbiness to make sense for reproductions (but are awesome for curtains...). Check the decorating section in addition to the apparel section. They do have limited wool in stock, as well.
Reproduction Fabrics: Hello, lovely authentic prints. I'm Hyaline. I'm going to buy all of you. This site is wonderful for finding period-correct prints--something that's almost impossible to find in most fabric stores. Careful--the 1775-1825 section will have prints that are great for one end of that spectrum but not the whole time. Still--in.love.with.this.site.
There are tons more out there--and don't even get me started on trims and tapes and ribbons. Have fun fabric shopping...and share your favorite sources in the comments!