Instrument Purchase, Care, and Repair

Brass Resources
You'll find on this site a large collection of links, ranging from silver-plating companies to pages on music theory, from articles on valve oil to practice routines. Most links are to sites that sell some kind of brass-related accessory, or that offer repair services.

International Horn Society
Horns for sale.

Raw Brass
This buyer's advice site has a large collection of links to makers of equipment for brass instruments. It classifies brands in alphabetical order, and clearly states the specializations of individual makers.