Using a dc power supply instead of batteries
Unfortunately, just a 6 V supply won't do it.  You really need a &plusmn;3 V supply.  Or, you can get two 3 V supplies and connect the + of one to the - of the other.  That common connection will be the ground of your circuit.  The remaining + and - ends are the +3 V and -3 V ends.

The latter is probably easier to source.  Basically, get two cheap 3 V wall warts.