Before you mirror, check the names of the nodes to make they have a consistent prefix (e.g. L_ for left side) and descriptive names (e.g. R_foot_control).

Select the bones to copy and click the Mirror tool on the Modify tab of the shelf.

In the operation controls toolbar , set the New prefix and Remove old name up to fields to automatically rename the nodes as you mirror them and set the Mirror Plane.

For example, if your nodes start with L_ and you want the mirrored nodes to start with R_: