Re: I'm guessing, but ...
"fitting a second hand unit over 10 years old"
That shouldn't be an issue. If you didn't have an accident that set off the airbag you are replacing, it would still be old. A "second hand" unit from another car that had never been deployed (similar year, make, model, etc) should work just fine.
At the rate insurance companies "total" cars, any car that has been in an accident is likely to be scrapped so there isn't a market for third party replacement airbags for older cars. I ran into a similar problem with a catalytic converter. I needed a new one for a 1986 Mits pickup and was told I'd have to get one that met 1998 standards in California (thank god I moved). They weren't made. What they were telling me was that I would have to scrap my whole truck. No way, José. I had one shipped to a friend out of state (vendor wouldn't/couldn't ship to California) and sent him the money to send it on. Worked just fine, passed the emissions test with flying colors, so what's the problem?