I'm visiting my inlaws, who are elderly. The plastic soap dish insert
in one shower is broken. I'd like to find a replacement, but haven't
found a source. I'd appreciate any suggestions.

Here's a photo of the broken insert with dimensions.

http://tinypic.com/r/1z383vb/7

Here's a photo of the base that is embedded into the wall of the
shower, which is one of those one-piece units made of something like
fiberglass.

http://tinypic.com/r/mu7n5u/7

However, I would rather not replace the base unit because it will
probably not arrive before I leave and they will then have to hire
someone to replace it. I'd like to buy just the plastic insert.
Unfortunately, the shower is probably 25-30 years old.