Can anyone tell me if such a radio can be purchased from mustang online websites or do we have to do something custom like datac has done?
No, no, no, don't do that to Gary. As I've explained before, he won't have the foggiest idea what you're talking about- he does radio conversions, period, no mp3 function or anything else. I used the aux inputs in the AAR conversion board he installed for my mp3 board.
I bought the board (several, actually) from Jameco Electronics, IIRC it was well under $10. After over a year's worth of heavy daily use (the car's a daily driver), it's held up great and I couldn't be happier with it.
As I mentioned above, my initial plan was to buy a bare radio conversion board and install it myself, but (as you've discovered) unless I was willing to buy a case of boards or pay $$$ in markup to a sketchy dealer (he actually wanted within $20 of what Gary charged, installed), I was out of luck. BTW, Gary's now using a different board, which he claims is superior, but has some differences in how the aux input switching is handled. The AAR board (which is still available from other vendors) ships in an autoswitch mode, and switches from radio to aux when sensing a signal on the aux feed. I ran into a problem where it would switch back to radio during long quiet mp3 passages, which didn't make me happy. With the AAR board you can remove a component on the board and add an inline diode which, when 12v is applied, switches to aux input- it was a simple matter to connect this with the power source for the mp3 board, so that when the mp3 board's powered up it's in aux mode.
Gary does technically excellent work, but I did run into a painful and frustrating customer service experience with him afterwards which needlessly cost me both time and money. I would most definitely hesitate before asking him to do anything, and I do mean anything, beyond his standard conversion, which, as I've stated before, is first-rate.