Well you can get -9v out of +9v if you are willing to use a Boost converter connected as a minus voltage generator. That requires a switching type power supply circuit. How hefty it has to be depends on how much output current you need, and how much is available from the +9v supply.
From what you are saying i think it would be better that you post your circuit like Eric mentioned, and also tell us what kind of power supply or battery you are using to power it. The reason for this is because there are a huge number of ways of doing this with the circuit complexity ranging from a couple resistors to a special kind of IC chip with an inductor and several capacitors. If we know what you want to do we can make a better recommendation.