Power source for a HDMI to A/V adapter for streaming services.

We have a large analog TV with no HDMI port so we acquired a HDMI to A/V adapter. The adapter shown in the photo accepts the HDMI cable to the streaming device (be it Roku or Fire Stick). The seller of the adapter had a photo of a power cord to be plugged into a 110V wall outlet but the adapter only came with a USB cord. And the seller stated that in the description it only mentioned the USB cord and no refund or replacement of the part. 

Where the cord connects to the adapter it says USB Power. I hoped to use one of our cell phone chargers but the end of the plug does not match the "mini" HDMI port on the side of the adapter. Our computer has 6 USB ports (4 rear for two printers and keyboard and mouse and 2 for camera and cell phone uploads of photos and videos).

I have a USB extension. Could I use that to go from the end of the adapter cable to a power plug and on to a wall outlet? Since the seller's photos showed a two prong power cord I assume that it would not hurt to use their USB cable and extend it using an extension cord and on to a wall outlet. If the 110V is not frying the cameras and phones it should not (??) harm the HDMI to A/V adapter.

If we need to use our computer to power the adapter which of our 6 USB ports would be a power source? Thanks.