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Use a switch mode regulator to generate it, I seem to remember the MAX774 being a -5V generator. Depending on what you are planning on doing it may be advisable to follow the output with a 7905 linear regulator
If it simply an op-amp then simply a virtual ground!
Have a look at DC/DC charge pumps converters ..
Pretty common configuration is Vout= - Vin ..
LM2662/3 will deliver -5V @ 200mA when supplied of +5V ..
MAX660 --> -5V @ 100mA
ICL7660(MAX660) --> -5V @ 20mA .. and so on ..
You can also consider generating -5V from +9 to +12V with 555 timer (see picture below, both diodes 1N4148, or similar)