Definately a scrapyard challenge

[Jonathan]

Using a timing belt is a great idea for a number of other reasons. It enables the ratio between the stepper motor and ballscrew to be changed. This alters the region which the motor operates in within the speed/torque curve, which with careful selection will get better acceleration and or feedrate. For instance I found with my Z-axis by calculating to get an estimate and then trying various pulleys that 42:22 ratio was best, with the larger pulley on the motor. The other big gain is the timing belt 'seperates' the ballscrew and motor so the vibrations do not transmit so easily - in other words it damps the system reducing resonance. Again this improves the feedrate, so it's a good thing to do on the X and Y-axis too. If you put a 2:1 ratio on the X and Y ballscrews it should get better acceleration and less importantly top speed, but you sacrifice resolution slightly (in most cases negligibly) plus you've always got the option of swapping the pulleys round to get better resolution...
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[Jonathan]

Last time I checked cnc4you was cheapest for steppers including postage (though it looks like that may have changed now) and eBay for the drivers. I'm not aware of any other places.

The drawing in post #98 - You should make the joint between the X-axis linear bearings and gantry sides stronger. Currently it's only aluminium angle which will bend at the corner relatively easily, and since the bearings are free to rotate upon the rails that will cause deflection. Perhaps you could use box section there, or better just add some triangular pieces to stiffen it.

Power supply - 500VA toroidal transformer plus bridge rectifier and capacitors is all you need. www.rapidonline.com are reasonably priced.