I lost a book some years ago that I started reading about an island inhabited by creatures / animals that had vital organs that were exact dna matches to humans who could afford the price. The crux was they were only animals. During course of investigation, it is found that animals were problem solving. I'd really like to finish this story. I thought it might be "the island", but the one I was able to find on line didn't match the story line. Any help is appreciated.

Hmmm - this novel has bits of what you described - animal organ harvesting, an island, but I'm not quite sure it's what you're looking for...
Ancestor - Siglerpedia

Another possibility
Robin Cook's,...

Chromosome 6

A group of geneticists find the gene that can
be transplanted into apes to insure the apes
organs would provide perfect matches for humans.
Unfortunately, the bit of genetic material also seems
to be the bridge between animal and true human
sentient thought.
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That's IT! Chrosome 6 by Robin Cook! Thanks so much!!