Do you have regular access to a gym or the like? They should have a pretty accurate scale around, like the one at your doctor's office.
These scales are only accurate if they get calibrated at least once a year. Look for a calibration sticker. I doubt you will find one. Even the doctors don't bother getting it calibrated.
I have this one. Cheap and very accurate so far(3 years). I never get a different reading as long as it is on a hard level surface.
http://www.performancebike.com/shop/...tegory_ID=2418