Alpina Manufacturing has a good range of products that allow you to display graphics on walls or stands very easily, and the frame easily opens to allow you to insert a graphic - and they are economical.
Displays2go.com is one of my default places to look for sign frames, iPad holders, and even light boxes. You could probably outfit an entire booth from this company.
For additional Slim Light boxes check out:
Blue River Digital
Note that depending on how you are using the light box you might need to check out the fabrication. Most thin boxes lit by wrapping LED's around the perimeter. On many smaller sizes this is fine. And if you engrave the Acrylic it produces a really nice effect, though obviously different than lighting a graphic. On larger boxes, places light environmental lights custom builds the light box with proprietary engraving patterns on the real of the acrylic to ensure that the light box is evenly lit throughout the whole surface of the product.