My favorite site at the moment is Anvil Industry, run by Joel Pitt out of London, UK. I wrote to Joel months ago asking him to make some combi weapons. He delivered by making some magnetised combination weapons for his miniature range. I'm sure he's had plenty of requests for the items before I came along, but it felt like he really listened to me. I want to add that the quality of Joel's products are excellent and well cast. I've picked up some bitz from him in the past, and continue to look forward to what his company will be putting out in the future.
I'm dusting off my Tau army now since it's the flavor of the month. I turned to Paulson Games a company located in Downers Grove, IL for extra bitz to customize my character miniatures, and to add a little flavor to the rank and file. The quality of the products are good with no heavy mold lines or miss casts.