While I don't work at an electronics store, I do have a degree in the subject. There are some basic properties you should know about concerning the connector to your phone, and any phone that uses a USB connector (mini, micro, A or B). The USB standard, which is what the HTC ExtUSB connector is based on, limits the voltage on the connector to a maximum of 5.25VDC. Current is a maximum of 1.5A. Understand that electricity is like waterflow, basically. Voltage is the same as the pressure in the pipes and current is the same as the amount of flow. Too much of either will burn up your electronic components.
The wall-charger that came with your phone outputs 5VDC @ 1A. Any car charger that has the miniUSB connector will output the same voltage, but the current is what determines the amount of time it takes to charge the battery. I use a standard USB-miniUSB cable and plug it into a USB adapter that I bought at Autozone. If your kids have car chargers for their NintendoDS or PSP you probably already have one of these. They have standard USB connectors on the end instead of a cord with a connector on the end. These devices automatically drop the output voltage to 5VDC to adhere to the USB standard.
I have a Mogul and my wife has the Touch. She used to have a RAZR and the charger has the same miniUSB connector. The Moto charger works fine, but only supplies 550mA (.55A) so the charge time is a little longer. The cable I use doesn't pull quite a full amp, but I am in my car for long periods of time so a slower charge still gets me charged in full for the day. I prefer a slower charge anyway, to extend the battery of the Li-ion battery. I also use the Moto chargers at home because I've bought 4 of their BT headsets over the years and, again, they have the SAME CONNECTOR. Also, a lot of the iPod car chargers are basically standard USB cables and cigarette lighter adapters.
Pay attention to the current output of the charger you use and basically the more current it puts out, the faster charge time. Again, the maximum current that a USB connector will push is 1.5A, but your phone is rated take 1A only.
BTW, a "good power supply" is one that provides clean DC voltage without letting AC ripple or noise intrude on the voltage line. Also, you don't connect devices to a power supply in series; devices are plugged into a power supply in a parallel configuration. If it's in series, that means each device's current draw would have to pass through the device before it and inevitably all devices would be technically plugged into the very first device. If you did that, the first device would basically be passing ALL of the current that the other devices use.