+7.5V/+15V/-15V or similar would be ideal but I've
never seen one
available.
This is something VERY easy to get with a PC power supply and one pot or
one resistor.
ALL PC power supplies uses a (or a variant of) TL494. Change the divider
network (two resistors) on pin 1, connected to 5V and you can raise it to 8V
and 12V will have around 18V
Simple, easy and stable.
Greetings from Brazil
Alexandre, PU1BZZ