Answer:
I recently built a food smoker out of a food grade barrel and wanted to heat it using propane. After researching the Internet and pricing out parts, I found the easiest way to get the propane burner was to buy a cheap old gas grill off of Craigslist and salvage the parts I wanted.
Basically all you need to do is take the fittings off of the grill and find a way to attach them to the inside of your barrel or smoker.
I then bent up a small wire rack that holds a pan for my smoking chips, with the burners right underneath it. I've been able to get my 55 gallon drum up to 300 degrees or more.
Long story short, they're hard to build, easy to salvage.