Where's the best place to get a capacitor?

not non-polar electrolytic cap

If your loudspeakers are of quality and you hope to use them for a long time ,
then I advise you do not buy electrolytic caps for repairs , because they will deteriorate ... as you can see the one you have has.

Buy polypropylene , and a low voltage one will suffice.
You will not need one of the 400 volt rated capacitors to replace a tweeter capacitor.

If the sound is slightly too bright with the new poly cap , then connect a 1 ohm 5 watt resistor in Series with the capacitor.
Electrolytic caps , such as your old cap seems to be , have internal resistance ,
and polypropylene caps do not ,
but listen first , because you may prefer the sound with no added resistance.
You can fine-tune the treble output with slightly more or less resistance.
If the original sound had OK treble balance , I doubt you will need more than 1 ohm added , but you may prefer a little less added , try 0.47 ohm next.
This will have to be decided by listening.

I advise you buy a new 6.8uF cap for the other speaker and install now also.
You will then have same treble quality from both speakers ,
{ and the other old cap will fail eventually }.