I just sold a Squarebird with an 1980's FM converter a month ago.
You might find used ones on some auction sites.
I agree with Casey, there are better options for you listening enjoyment now. You can replace your stock radio with one that fits in its place or you can install one of the new Secret Audio that keeps your old radio, but allows you to hear FM and CD's.
I made my own secret system years ago. The radio looks stock, but it has a stereo hidden behind the face. The original volume control still controls the volume of the stereo. The CD player hides under the front seat. The judges are totally unaware the car has a stereo.
1960 Golde Top