Really, it's a very small issue - as long as the 5v is decoupled you should have no problem - especially with the 78L05 option, which is really a separate power supply anyway, just that it gets its supply from your 24v line rather than the mains.
If you really must have a separate supply, if the mics will run at 4.5v, you could run them from 3x AAA batteries (or 4x if the mics will run at 6v), or even a stack of button cells. The mics will draw hardly any power so they should last a long time. Better than building a separate mains power supply.