I bought a large one years ago from an art store, thankfully I still have it. Now they cost close to $200 for that size.
My friend uses a drawer from her cutting table. She opens the drawer, puts a foldable Ott lamp in it, then a piece of plexiglass over the drawer sides. Works fine. Any box that you can get a light source into, and then place glass or plastic over the top will work. Or, you can buy one for the cost of a bolt of fabric from an art store!