RS-485 isolation required?
You can connect the two grounds if they're truly floating wrt each other. Then the CM range is controlled and you're back to the original situation. Several volts of difference is okay (-7 to +12) so any reasonable connection over a short distance should meet this requirement. A high CM range transceiver would still require some kind of control of the common mode voltage. 

Isolated DC-DC converters are very common, just do a parametric search at some distributor such as Digikey. I see 176 pages (25 per page) of different isolated DC-DC board-mount modules before filtering for required power and for input and output voltages. 

Edit: One of my favorites is the TI (nee Burr-Brown) DCR01 series, which has an LDO (no problems out-of-spec voltages with light loads) and fits in an SOP that is 2.5mm high. It can supply 1W, so 200mA at 5V.   

There are also RS-485 hybrid transceiver modules with the DC-DC converter built-in, but they're a bit expensive, and the DC-DC converter is probably a bit inefficient compared to one with a less constrained transformer. 



You should connect the isolated ground to the ground on the remote system to control the common mode range.