@zrb IANAL. I contact the vendor's customer support. They might state that without a service contract they are unable to help you; that doesn't matter and you should inform them of that. Be polite but firm.
GPL only requires someone provide source code within 3 years of last distribution. So if the firmware is older than 3 years (2022 now) you may need to prove that they're still distributing it (e.g. give them a link from their own website to download the firmware)
"""
I own one of your devices which contains a firmware incorporating GPL licensed software.
As you are distributing a firmware with GPL licensed software on these devices, you are obligated under the software license terms to provide the source code upon request [1]:
> b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
The acquisition channel of the device has no bearing on the validity of my request or your legal obligations to fulfill it.
I am happy to provide the serial numbers of the devices in question to prove my ownership.
"""
They might ignore you or refuse, and then your only recourse is to contact a copyright holder (e.g. busybox) and hope they have the appetite to enforce GPL, which in my experience they usually don't.
Also, and I hope this is obvious, you need to first verify that the firmware contains GPL software like Linux, busybox, U-Boot, etc. Some devices run a custom RTOS or Wind River. Don't bother vendors for source code unless you're certain they're shipping GPL code and don't offer the source code download.
