Megaperls has new and used pa45 backs from time to time. They are also showing a "145" back now, not sure what the difference is but have a look... they have used ones too...

http://www.japanexposures.com/shop/i...0623d4ac75cae6

If all you want to do is check exposure of a central subject then you can use a smaller format (3.25x4.25) instant film in the 405. That saves some $$$ on film.