Where is the center point for the Supergalactic coordinate system?
According to Wikipedia, the origin of the system is:

The zero point (SGB=0°, SGL=0°) lies at (l=137.37°, b=0°). In J2000 equatorial coordinates, this is approximately (2.82 h, +59.5°).

In the same article there is a recent referred paper on this topic, which explains [The zero point] is one of the two regions where the SGP is crossed by the Galactic plane.

Perhaps you already know this website, which shows many good pictures. I was trying to find a better figure, but I guess you have to push your imagination according to the definition.

EDIT: According to this, SG coordinates are similar to the galactic ones, so that the North Supergalactic Pole is defined in Galactic coordinates, which means that they have the same origin (i.e., the Sun).