To make a cheap IR filter buy a roll of 120 size E6, and have them develop it as is. It'll come out as a wide strip of dark plastic. Cut out the size of filter you need and attach it to a UV filter or something. It blocks most of the visible light, but not so much IR. To get even more effect use a double layer.
Put a double layer of developed E6 in a mini softbox on your flash, no filter on the lens, and shoot IR flash photography in low light. If you tape up the softbox good enough there will be almost no visible flash, but plenty of IR flash.