If anyone is interested in trying modular construction, I buy my materials as surplus offcuts straight from the manufacturer. By carefully looking through their listings, and being just a little bit flexible in the size of project you're building, you can put almost anything together.
They have different series of materials, and also standard & metric materials. It's important to use all materials from the same series. I'm using 10 series for most of my projects.
It's also important to carefully plan out your project, and have a complete build list before ordering materials. As stated, these pieces are all surplus offcuts. A length of a certain material may be available today and gone tomorrow, so know exactly how many pieces of a certain length you need, and order them all at once.
The company I have been buying my materials from is
http://stores.ebay.com/8020-Inc-Garage-Sale/Extrusions-Fractional-T-Slot-/_i.html?_fsub=6479546
