Try Fisher Scientific. They sell a variety from research grade to teaching lab grade. You may also wish to see whether your local college periodically cleans out and sells old equipment. Occasionally a mechanical Mettler balance (with the agate knife-edge) will show up, as they're considered too masochistic to use these days. I have one from a previous lab demolition,and it's a nice balance, though the 0.0001 accuracy is probably more than is needed for mixing cyanotype.