There are Clipper machines at "virtual OAK station"
The BART-OAK cable car* is free to simply ride, if your mission is to buy a Clipper card and immediately return to OAK. The fare gates and Clipper sales machines are at the Coliseum end.
It's quite fast, and you could do it while your companions collect luggage.
BART is a variable-fare system, meaning you must card-in when you enter, and card-out when you leave. However, the OAKland Airport cable-car is a simple 2-station shuttle, and the Coliseum end empties only into the BART station at platform 3. There's no way to get landside from platform 3. So BART cheated. Since there's nowhere else you can go, they moved the OAK fare-gates to the Coliseum end.
Since the fare-gates are there, so are the ticket/Clipper sales machines.
Riding between OAK and platform 3 is free. The agent told me that's how you joyride, and that's also how you buy BART or Clipper tickets if you're at OAK and don't want to ride BART that day.
How does money work, then? If you pass through the faregates at Platform 3, you are deemed to have entered at OAK station (since there is no other way you could get here). The cable car goes from "free" to about a $4 premium over normal BART rides.
* Yeah. It's really a cable car. Not quaint, but modernistic because BART. It doesn't have a grip or gripman, and the cable is permanently bolted to the car, unlike the system across the bay. The cable stops and starts to move the car. You can watch the sheaves turn at the OAK end; that's the powerhouse.