First generation aftermarket stereo

There's a chance you may find one. The real challenge is the existing stereo is so integrated (at least in Gen2 Prius) that you have to install a resistor or put the existing stereo under the seat in order to male everything work correctly. That might not be the case in Gen1, but see next commenters to confirm.

 

A bigger issue for Gen 1 is that the dash center platic piece gets extremely brittle over time. To the point of it just cracking into many pieces and crumbling away when you try to pop it out if the clips that hold it in place.
Then your dash will look terrible and parts of it may even become non-functional (like the vents).

New replacement piece is a few hundred dollars from the dealer.

There are those that claim that they can get the gen1 dash out without breaking it, but I have never had luck on any for a long time. They are just too old/brittle.