Assuming you don't care about code and it's temporary, I would just get a piece of scrap metal and make one. Use tin snips or something to cut a circle about 10 inches in diameter, put a few legs on it and mount it on top of the pipe. I'd worry about flow bring cut down with two elbows. I mean it'd probably be fine but just in case. If it's a wood furnace I'd also worry about soot build causing a fire.