+12V and -12V for supplying Op-Amps in my circuit
I've tried using 2x9V and 4x1.5V batteries in series with a voltage
  divider.


You don't need a voltage divider. Just connect 4 sets of 4x1.5V cells in series and make their midpoint the ground, like this:-







simulate this circuit &ndash; Schematic created using CircuitLab

When connecting batteries in series always use the same type, and don't mix old and new cells. When any of the cells go flat the others will soon follow, so replace all of them at once. If you use standard AA cells and buy in bulk they may be cheaper than any other solution.