I've found some at a Reebok factory outlet store. I've also ordered some on the internet (I searched for "weighted gloves" or maybe even Ironwear gloves, because I love the way they fit and feel (the ironwear "softiron" gloves. I've tried other types and they were either too hard and irritating--Walmart gloves--or too "shifty"--sand-filled gloves).
Definitely start with 1#, and don't use them through the entire workout at first. And don't use them until you are sure you are doing the punches with no lock-out and great form, as the extra weight will exacerbate any problems you might develop from less-than-perfect form.