A good place to buy a digital postal scale is EBAY. Go there and do a search for "digital postal scale" and you'll literally have hundreds, if not thousands to choose from. For the best value look for one that offers Free Shipping. Here's one that weighs up to 86 lbs. with accuracy to .2 oz. offering more versatility than for just weighing guns.
http://www.ebay.com/itm/Accuteck-Dre...item5d4a9c2ed2
These typically sell in the $6 to $12 range and are delivered free to your mailbox or front door.