How much power do you need in your isolated circuit?
Probably you should use the isolated DC/DC converter.
There is quite a big number of them available on the market.

Just as an example: [offer from Farnell][1].

You must also be aware what should be the quality of isolation (for the safety reasons). However in case of lightnings you may need additional surge protection.



  [1]: http://uk.farnell.com/dc-dc-converters-isolated-board-mount