Not sure how much current you need, but a typical rs232 transceiver (e.g. MAX232) generates +10V and -10V from a single +5 supply. If you tap it at the right place (on the correct charge pump capacitor leg, or one of the output pins if you tie an input low) you can get ~ +10V.
Not sure if you'd want to use +/-10V to drive an op-amp, or if you can possibly PWM the MAX232 input pin in some way to get +/- 10V PWM on the output pin.
No idea how stable or clean, but it may be worth a look.
-j