Many musicians are known for their improvisations but was not always recorded. I would like to point out to you an attempted transcription made long after the death of the musician.
Rekonstruierte Improvisationen über volkstümliche Themen
The title means Reconstructed improvisations on folk themes. This recording was made after archives performances by Pierre Cochereau. He was organist at Notre-Dame in Paris, he was known to improvisates after themes. He never written music scores. The transcriptors names are indicated.