Have you considered two 15V modules in series?
I also need -15 Vdc. Not sure I understand.
If you take two single voltage 15V modules, and connect +Ve from module 1 to "+15V" on your consumer, then join -Ve from that module to +Ve from module 2 and "GND" on the consumer, and finish by connecting -Ve from module 2 to "-15V" on the consumer, you've built a dual voltage supply.
The
important assumption here is that the
output on the modules must be floating w.r.t ground/chassis/mains PE. Most Mean Well supplies for instance solve this.
There's an UK seller offering new 15V
Mean Wells here -- they've only got a capacitor between -Ve and GND which likely make them OK for your application, as long as you're not floating too many Volts away from GND.
Then of course, I'm no EE and my guess is as good as yours. But it's a start.