DIYs and How-Tos:

How to put up a Deluxe 12 Gourd Rack (NCPMS youtube video)

Martin Products:

PMCA shop (various houses, gourds, gourd racks, and accessories)
Uncle Blaine's Sparrow Trap (repeating ground trap)
The Supergourd (Bird Abodes)
S&K Manufacturing- Gourds (BO11P, BO-9 gourds)
Troyer's Birds Paradise - no website - call for catalog - 1-800-872-0103 (inventor of T-14 and Troyer Horizontal gourd, etc.)
Purple Martins R' Us (various martin products)