You must use a popper in which the hot air enters the popcorn chamber from side vents. 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, since chaff will get stuck underneath the mesh and become a fire hazard. The correct type of popper swirls the chaff upward and out of the popper. We have heard from customers that successfully use a mesh-bottom popper but we still don't think it's a good idea.
We have tested a LOT of poppers, and still prefer the discontinued West Bend Poppery II. You may be able to find them at thrift stores or through private sellers online. It has the best chute design for home roasting, and is rated at 1200 watts. We think the next best thing is the West Bend Air Crazy which we sell on our site. 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 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!