USB power from multiple sources

I need to drive a higher power device that communicates on the same lines as the USB.

what I want to do is make a USB cable that has external power, and then feed both the Arduino and my device from that cable.

I was hoping to find something I could just buy, but it appears that this is not a mainstream need and no one mass produces it.

any suggestions on what to watch out for ?
maybe where to purchase one ?

Suggest you buy a "USB extension cable" that has type A male and female ends. Cut that to get access to the 5V and Ground conductors.

Apply 5V with some high current rating like 2 amps, maybe from a supply like THIS:

That could feed both the Arduino USB and your other device...