Amazon.com, eBay and The Barbecue Store are a few of the places to buy a gas burner diffuser, as of February 2015. Diffusers can be found for as little as $7.88.
The Barbecue Store sells diffusers specifically for outdoor gas burners such as those found on camping stoves and barbecues. While the company is based in Europe, it ships internationally, and the diffusers it sells are made to fit all types of outdoor gas stoves. The diffusers available on eBay and Amazon.com are created for indoor gas burner use. Prices depend on the individual seller and the type and make of the diffuser.