The SBB Travel Office is just above the Zurich Airport train station. You will walk by on your way to the platforms. Simply follow the signs “Bahn/Train” when leaving the baggage hall. It is all well signposted.
You can find the opening times here: Station Zürich Flughafen | SBB
You can, of course, buy the STP from the SBB Travel Office. Keep in mind that this will be a physical ticket, if you lose it, or it gets stolen, there is no replacement. While the electronic version is a PDF that can be reprinted as many times as required, or even be presented from your mobile device.