They've got a real cool utilitarian vibe to them. Some have a tattered patina, others are shiny. It all depends what you're looking for. Sizing info below.
Tapped to your specs, from 5/16-18, 5/16-24, 3/8-16 or 3/8-24. Can do 8 and 10 mm sizes, too. $25 ea., while supply lasts. Once they're gone, they're gone.