If you want to buy a new head unit you could buy a single din Alpine unit and then get an instalation kit that would fill that gap. Most, if not all, car stereo places carry, or can get them. Or you can take your Supra to your local stereo shop and have them fit your 1.5 din into the supra if you really like that H.U. Or..if you're adventurous you could build it yourself from ABS plastic or Fiberglass.
If you want a new double din alpine, I belive the CDA-w550 will fit.....Not positive though....