It’s a guesswork without knowing what the load current is ..
For small currents you may consider NMAxxxx dc/dc isolated converters ..
For example, the NMA0515 delivers +/-15V out of +5V ..
Or you can use NMA0509 (+/-9V = 18V ) followed by LM317L (18V-to-15V) to produce regulated, isolated 15V ..
Without current, it really is guess work... look at the TRACO website, they have really nice isolated DC/DC with all the specs... Home : tracopower.com
The TES1 series is nice, it's SMD and it's always distributed by Farnell or Radiospares..