I use one called Good Cook. I think I got it from Wal-mart. It's designed to weigh food so it comes with a square bowl that an adult dragon fits in, but if he is fiesty you'll want to put hi in a pillow case to keep him still while you weigh him.
we bought a digital postal scale, & use a small rubbermaid tote to put our dragons in. Most digital scales have a tare button, put your empty container on the scale, push the button and it will zero out, then put your dragon in & it shows actual weight of dragon only. No math needed that way, makes it really easy.