thanks everyone, those ATX cable adaptors look like a good idea and i had forgotten about the ATX Flex PSUs, i used one a while ago when i replaced the PSU in my old Atari TT so i am not sure why i didn't think of it. In this case it's probably a good option given i also need to power the hard and floppy disk drives.
i've not looked at the actual 5v power consumption of this 386 but the entire system (including floppy & hdd) only draws 33 watts right off the mains. It also seems that the -5v is not being used anywhere, the i/o card does use the -12v though, presumably for the RS232 serial ports.