Building a good-enough optical table relatively cheaply isn't too hard. Use cinder blocks for legs and then alternately layer plywood and carpet, with a layer of spaced out wheelbarrow inner-tubes midway through. You can even add a final layer of sheet steel if you're using magnetically mounted optical components. You can test the stability by reflecting a laser-pen off a dish of water and watching the reflection on a piece of paper/wall. Most important is to be away from traffic etc...