Double-wall type B vents are available at Amazon.com and HomeDepot.com as of 2015. Amazon.com offers a Metal-Fab 4-inch-by-36-inch vent for $14.99, and HomeDepot.com sells a Speedi 5-inch-by-18 inch model for $11.50. Other sizes and dimensions are also available through HomeDepot.com.
Both the Metal-Fab and Speedi double-wall type B vents are UL listed for use with gas-burning appliances. The Speedi vent features twist-and-turn snap fitting. Its double-wall pipe is composed of a galvanized steel and aluminum exterior and an aluminum-lined interior. The Speedi model has a maximum gas temperature of 470 F and must have a 1-inch clearance from combustible items.