I figured a resource post would be a good inaugural post, so here's a list of those companies I know of that sell vintage patterns and new patterns from vintage designs. If you know of any that I missed, leave a comment and I'll add it.
Modern ReproductionsButterick Retro - Butterick Retro is an imprint of Butterick sewing patterns. They appear to be reprints of 1940s and 1950s Butterick sewing patterns, but not having used them myself, I can't say much more.
Decades of Style Pattern Company - Decades of Style sells reproductions of vintage sewing patterns from the early 1900s through the 1950s. Their patterns include all sizes from Bust 20" to Bust 46", and are printed on heavy tissue paper. Their instructions have been updated, and are clear with ample diagrams.
Folkwear - Folkwear has been around for quite a while. They have a selection of "Retro" patterns, as well as clothing patterns from around the world. Older Folkwear patterns are printed on heavy paper, but newer ones are printed on tissue. Their sizing tends to run a bit small, so make sure to double check against your own measurements. Their directions are usually pretty clear, and include plenty of diagrams.
Past Patterns - Past Patterns sells reproductions of patterns from the 1700s through the 1940s. I haven't used any of their patterns, so I can't speak for their instruction, but please note, all Past Patterns come
only in one size: the size of original pattern they are copying.
Vintage Vogue - Vintage Vogue is an imprint of Vogue Patterns. They are reprints of Vogue sewing patterns from the 1940s and 1950s. Once again, I personally haven't used these, but I assume they're pretty much like Vogue patterns, which tend to be some of the best commercially available sewing patterns.
Vintage PatternsBlue Gardenia Vintage Patterns and Jewelry - Blue Gardenia sells vintage patterns from the 1920s through the 1960s. They have a large selection, but they also tend to be really expensive: $35-$45 is the low end of the range. Even incomplete patterns are around $20.
Patterns from the Past - Patterns from the Past carries sewing patterns from the 1920s through to the 1990s. They have a good selection, and their prices are lower than those of Blue Gardenia, but still around $20-$30 for an uncut pattern or $10 for a cut pattern.
So Vintage Patterns - So Vintage Patterns has a large selection of patterns from the 1900s to the 1970s. Unfortunately, they're just as expensive as Blue Gardenia - you'll be lucky to find a pattern under $35.
Vintage Martini - Vintage Martini has all sorts of antiques and vintage stuff for sale, including sewing patterns, from the Victorian era up to the 1980s. Their site design is terrible, and there is very little information on the patterns, but their prices tend to be around $15 per pattern.
Vintage Pattern Lending Library - The Vintage Pattern Lending Library, as its name suggests, is a little bit different than most vintage pattern sites. According to their website, "the Vintage Pattern Lending Library preserves, archives, and replicates historic fashion patterns from 1840 through 1950, vintage sewing publications, and fashion prints of the past." They do offer some patterns reproductions for sale, though only in the same size as the original pattern. The prices are very reasonable.
OfflineI have found the majority of my collection of vintage patterns through second hand and antique stores, usually for much, much lower prices than those found online! Check out antique stores for vintage patterns, as well as notions like antique buttons or detachable collars. You can also find cute things like antique needle-books, scissors, and whatnot. Stay away from antique thread, though, as thread usually weakens with age. Thrift stores might have patterns, and second-hand craft stores are a good bet.
Pattern ReviewsThe Great Pattern Review - The Great Pattern Review is a wonderful resource for those people who like to use reproduction or historically accurate patterns from strange little companies no-one has ever heard of. There are scores of reviews, some very detailed, by people of many skill levels. I really like this site!
PatternReview.com - PatternReview.com is mainly focused on reviews of the big commercial pattern houses - Vogue, Simplicity, McCall's, Butterick, Burda, etc - but they do have a fair number of reviews for Folkwear. You have to sign up to access some parts of the site, though signup is free.