You must use a popper in which the hot air enters the popcorn chamber from side vents. We know of three types of this design: The West Bend Poppery II, The Popcorn Pumper, and the Popaire 2 by Hamilton Beach. You may find others that are of the same design; JC Penny brand for example. Recently folks have said that a Salton model sold at Target works, as does a Toastmaster model sold at Bed, Bath and Beyond. But any popper that looks like the photo below will work!
DO NOT attempt to use a popper where the hot air enters the popping chamber through a mesh-covered orifice on the bottom!!! This design is not safe for home coffee roasting and could ignite the chaff. The correct type of popper swirls the chaff upward and out of the popper into your chaff-collecting bowl.
We have tested all the poppers mentioned above, and prefer the Poppery II. It has the best chute design for home roasting and is rated at 1200 watts. We don't sell the poppers anymore - a long story - but you might be able to find them at a hardware store, thrift store or discount stores like Walmart or Kmart. If you have time to visit a few thrift stores, you will probably find the correct type of popper. You MUST buy one with all the plastic parts intact and uncracked, or else the roast chamber will not attain proper temperatures.
You can't be sure that a thrift store popper will work: some poppers have two heating elements and if one dies the popper will seem to function but never reach correct roasting temperatures. Also, some poppers are equipped with a circuit that, if overheated, will permanently disable the machine. It's a safety feature designed to prevent fires. On the other hand, most thrift store poppers cost around $2 so you won't have to hock the record collection if you buy a dud!