Since the connector is proprietary and not a lot of 3rd parties have started making them they can be hard to find. You will have better results if you look for a cable fro the original TF101 Transformer as the cables are the same.
Here is one on eBay for about $13
You should note, that if you are hoping to charge your tablet you will be much better off buying an Asus wall charger since using the cable to charge by USB will take a very
long time and won't work while you are using the tablet. Similarly iPhone chargers and similar USB wall adapters will also be very slow and won't work while you are using the tablet.
You can buy additional wall chargers from Office Depot. I have purchased several and they have very fast shipping.
TF101/201 Power Adapter