The set of vendors of Linux-related products is diverse, growing, and certainly in flux. This is the sort of information where it would be highly worthwhile to store vendor information in a database and allow collaborative updates.
Note that I have not been doing much maintenance of this material of late, and as a result, some material is very likely somewhat out of date.
Yet another vendor of $1.99 Linux/Red Hat CDs.
The local LUG, NTLUG, of which I am treasurer, purchases supplies from Linux Central.
LSL is notable as the vendor that sells versions of the X library "Motif" for non-IA-32 Linux platforms such as Alpha and PowerPC.
LinuxLot helps Linux users find the best prices on Linux books.