Respectfully, it's not the volts - they are all 5V per USB 2.0 specification. PT's draw higher current, between 1.5 and 2 Amps depending on the resistance of the cartos (in the case of V4L, premium and wow carts are lower resistance than the cool carts and subsequently draw up to 2 A during use). There are a zillion "AC to USB" charging adapters, but many only put out 0.5 A as they only need to trickle charge ipods and phones, etc. This adapter sold by Amazon:
Amazon.com: IPAD-2USB: High Power Slim AC Wall Plug to 2 x 5V USB Adapter (Compatible with iPad): Electronics is an example of one rated for "high power" use, and states up to 2 A draw on the power supply.
Basically, anything advertised to work with the iPad will probably work, as it requires a higher current USB port. Another good option is a multi-port high power USB hub - especially if you can use the other ports to power other USB devices such as external hard drives, printers, etc. at your work desk / computer station. Google / search Amazon, you will find a lot of options - don't buy it if it doesn't specify the Amperage though.