ANY USB power supply will work, as long as it has the proper micro-USB end (so-called micro type B) for the iStick:
Of course, it should meet your local AC current specification (plug and voltage, accounting even cars with their 12/24 VDC lighter supplies...

), but usually all domestic supplies have a 110 - 240 VAC input, so it's more about the plug (USA, UK, European....), and, for better perfomance (less charging time) it should give at least 1,0 A (1000 mA) of output. So, with 1 A o more you will charge the iStick at 1 A (about 2 hours charging time from fully depleted battery).
The 0,5 A (500 mA) USB power supplies are just a thing from the USB specification, to which adhere most in-board chargers (like those on Vaporshark and Hana and alike), but it is now being re-written once tablets, bigger phones and other appliances (like iStick itself) are able to draw more than 0,5 A to suit its needs.