Where is the lateral position of the aerodynamic center?
For a simple tapered half wing the aerodynamic centre spanwise location is at:
$$
y_{ac} = {{b}\over{6}} \cdot {c_r + 2c_t\over c_r+c_t}
$$

where $b$ is the span, $c_r$ is the root chord and $c_t$ is the tip chord.

For a symmetric aircraft it should be at the centreline, since it's the combination of the two identical (but opposite sign) locations.

Source is a aicraft sizing code I wrote some years ago, I remember solving the integral for a simple tapered wing.
Looking quickly at design books I can find the following image in Raymer, hope this helps: