I had the same problem with my A500 sidecar (bought from Europe, couldn't source a US PSU). Wired mine to draw power directly from the 86-pin connector, it works fine as long as you're using a modern low-draw HD or any sort of CF adapter. It's just a couple jumper pads that need to be changed on the sidecar, Google should be able to find you the instructions (they're on the same page as instructions for A590), let me know if you can't find, I can try to dig them up. Much nicer than having an extra bulky PSU sitting under your desk also, IMHO.
