Where can I purchase a mini-vga to HDMI adapter for older macbook pro?
You can't.

VGA is an analog transmission standard, HDMI is a digital transmission standard. It's not just a matter of changing the connector, the signals they carry are fundamentally different.

You could look in to a USB 2.0 -> HDMI adaptor. Something like the ones on this page. Though how well those work depend largely on the graphics card in your Mac. The older Macs may not have enough horsepower in their graphics chipsets to drive a second display over USB at anything close to a decent resolution to use on a high definition television.