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.