New Age Enclosures - They will usually send you free samples if you ask.
Teko Enclosures - Excellent quality, and very generous with samples. Fast, free shipping on all samples.
Hammond Manufacturing - Versatile boxes, great service for adding custom machining to standard parts for moderate quantities. Transparent boxes make holes unnecessary for LED indicators, etc.
Envision Plastics & Design Uses NMR Technology (No Molds Required) to manufacture custom plastic enclosures for a variety of applications.
working with acrylic
"DIY Acrylic Refugium" describes how to make an acrylic enclosure.