In case you pursue the build-your-own approach, here is pinout of the A500s power connector:
http://www.amiga.org/gallery/index.php?n=3902.
If, like me, you're unsure if the image shows the connector from the inside OR the outside (mirror image), a meter to show continuity to the A500's ground plane is invaluable. Of course, if you're in the southern hemisphere, its opposite!
I've seen the square power connectors offered a few times, or, you can find a dead PS and steal the cable (BE sure to confirm continuity. Don't rely on wire color alone!).
