I can't even find that cap on Digikey.
The problem is not finding, it but choosing from so many. :grinning:
Amazon has em if I buy 100 of them...
Which is exactly what you should do. You will want them sooner or later.
Secondly, just trying to understand why the circuit cares if it's polarised or not.
In this case it does not as the voltage is always in the one direction, but it is very poorly explained so you would have to figure out which way it is (and in fact, the positive is the side that goes to the Arduino).
It does not matter as 0.1µF capacitors are mostly ceramic and non-polarised anyway - though you can get tantalum capacitors (polarised) of that value.
Turns out, the reason he found the capacitor was needed, is almost certainly because the circuit is laid out on a breadboard rather than a PCB, and he has not included the capacitor directly across the supply terminals of the op-amp.