Mental,
I've done a bit of browsing on some CNC/machining forums and was given some recommendations.
As I said previously, the most common use of a VFD is to drive a 3 phase motor from a single phase supply. (having 3 phase power installed is expensive!). There are however a number of low power VFD's (1/4, or 1/2 HP) available for use where voltage and/or frequency control (motor speed) are required. Prices for these are around the $200 - $300 mark. Perhaps cheaper on eBay although I couldn't see any single phase units listed.
The VFD's are programmable for voltage/frequency either via a built in keypad or in some cases via an RS485 controller.
Here's
one supplier of a number of different brands. Hitachi and Danross were both given thumbs up in response to my queries.
DIsclaimer: These are mains voltage devices and need to be installed in a suitable enclosure and wired up by a licensed electrician.