S&S Firearms is a vendor that is known throughout the hobby on reputation alone. Their forte is firearms - complete arms, kits and parts. The prices on firearms is on the hobby average, but S&S has everything you'd need short of powder and caps to maintain your firearm. You'll need to be patient for the catalog to download - it's broken into sections, but it's still pretty "heavy" and needs time to be downloaded.
As a testament to this vendor, most authentic groups have S&S listed on their sites. BUT do not confuse S&S Firearms with S&S Sutler in Gettysburg.