Need a part number for the Y axis stepper motor pulley (sd 3)

Part number will depend on the manufacturer. Not sure we know, as there's no reason for Solidoodle to publish this.
That said, the belts all come from SDP-SI, so I presume the pulleys do too, so with a bit of searching on their site you could probably find an identical one.

To the best of my knowledge the specs for the pulleys on the motor are:

* MXL 0.08" (2.03mm) pitch
* 18 teeth (please double-check yours though - mine are in the bottom of a box now thanks to direct drive)
* 5mm bore

(note that the pulley on the shaft has a 6mm bore - if you can't find one, you can always drill out a 5mm pulley!)

Given you're in Australia (like me!), eBay-shipped-from-China (while slow) is usually much more cost-effective than any of the usual American sites (McMasters, Misuni, SDP-SI).
I think this would work (if you count different teeth just search "mxl pulley"):
http://www.ebay.com.au/itm/111221932683

Good luck!