Specific Firefly Costumes
AbbyShot Clothiers sells a detailed replica of an Independent's brown coat via their website at http://www.abbyshot.com/products/ps-firefly-browncoat.php. The coat is fairly expensive; however, they have a number of pictures of the coat from the front, side, and back, so if you are interested in making your own, this is a good resource for reference photos.
For a less expensive coat that wasn't specifically designed to resemble a Firefly coat but looks reasonable, one fan recommends http://www.wwmerc.com/cgi-bin/category.cgi?item=CM88&type=store in dark brown.
If you're interested in making one, look at the duster patterns at Patterns of Time: http://www.patternsoftime.com/proddetail.asp?prod=EV53
FYI, the original brown coat was made by Jonathan A. Logan: http://www.jonathanalogan.com/
Inara's 1930's Dress from 'Shindig'
As Shawna Trpcic says in the commentary, the dress Inara wears at the end while sitting with Mal in the cargo bay with the cows is from the 1930's. The "1930's Tea Dress" on http://www.patternsoftime.com/proddetail.asp?prod=RH936 has a similar neckline.
Mal's Pants and Suspenders
Another couple of Browncoat-recommended links:
Check out these custom replicas of Mal's boots from MotorCowboy.com! http://www.motorcowboy.com/product/CAPT1
These boots also look the part: http://www.museumreplicas.com/webstore/ShowProduct.aspx?ProductId=200556
AbbyShot also offers a replica of Zoe's vest: http://www.abbyshot.com/products/ps-zoe-vest.php
Wash's Tropical Shirts
SamuraiX47 recommended the following as sources for tropical shirts of the style Wash wears, noting, "I noticed that Wash sometimes wears a vest (leather?) over the Aloha shirt and that under the aloha shirt is a collared polo shirt, and under that is a t-shirt, maybe black. Sometimes he's wearing a jumpsuit with the aloha shirt and vest over it."
Check out this discussion on sources for boots like River's: http://www.fireflyfans.net/thread.asp?b=13&t=9606
Blue Sun T-Shirts
You can get officially licensed Blue Sun t-shirt designs from http://stylinonline.stores.yahoo.net/serenity.html
Jayne's Orange Hat
Here's a discussion thread from Craftster, a fabulous website for people who like crafts with a twist: http://www.craftster.org/forum/index.php?topic=19076.20 (scroll down a bit for instructions)
Order a hat or a knit-it-yourself kit from http://www.wearwithstyle.com/
To trade tips with other knitters, check out http://www.livejournal.com/community/bigdamnknitters/
Jayne's Other Hat
If you'd like to find Jayne's black cloth hat from War Stories (see the picture), this is a good match: http://www.filson.com/product/index.jsp?productId=2092428&cp=2069836.2064999.2118241&parentPage=family
Mercsgoodgirl says, "The hat the merc's wearing in "War Stories" is definitely cotton duck canvas, not leather. My husband had a kahki one he picked up years ago at The Nature Company, which I dyed black. L. L. Bean's Aussie Crushable Hat is a close match, although you'll still have to dye it. Wash the hat in hot water and washing soda to remove any finish and use a concentrated hot dye solution. Remember to wear rubber gloves and use a dye vat you don't mind staining black. Once the color is deep and even, rinse and block (shape the brim and crown the way you want it) and allow to dry completely. As this may cause the hat to shrink slightly, purchase one that's a half-size too large."
Fun Fan Stuff
A running list of the various shirts, mugs, and other goodies made for Firefly Fans by Firefly fans can be found at http://forums.prospero.com/foxfirefly/messages?msg=19766.1