The outside of the fish appears fine so the issue was internal, probably an infection in the brain or body.
As Andy said, it's expensive and most vets will charge anywhere from $50-100+ for a necrosy of a fish, plus the cost of culturing if required. If you contact your local department of agriculture, they might have a fish health unit that can do it. Sometimes they charge, most times they charge businesses but do it for free for home aquarists.
A university biology department might do it, (especially if they deal with fish or do fish related vet courses).
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If you want to preserve the fish, put it in a small glass container with a 25% alcohol solution (have a lid on it). Leave it for 2 days, then tip the alcohol out and replace it with a fresh 25% alcohol solution.