There is a universal power supply from Radio Shack that is compatible. Enercell 12V/1500mA AC Adapter : AC Adapters | RadioShack.com
They will provide you a FREE tip to use with this adapter. The one you need to get for the a500 is Tip "G". Enercell Adaptaplug G : Adaptaplugs | RadioShack.com
It has a 6 foot cord so it is much easier to use the tablet while you are charging it. However, the tip is a little bit longer than the one that comes with the stock charger and it does not have the groove on the tip that helps keep the thing in place. So as long as you are not yanking the cord or swinging your tablet too much, it should work fine. I have used mine for a while now and it works just fine.
IMPORTANT NOTE: Make sure you set the polarity correctly when you attach the pin to the adapter. The "+" sign should line up with the word "tip".
I hope this helps. I hated the short wire on the stock charger. I now keep the stock charger at work and use this one at home.