They turn up on Ebay but usually cover several years Harley didn't have a book for each individual year. If you have a 78-1/2, you'll want a manual up to 1980 or 84 would cover you. I've got a 78-1/2 and a parts book up to only 78 and it doesn't have certain things listed but most parts were the same.
The latest manual you will probably find is the 1941 to 1984 FL parts manual. HD part number 99456-84B. The title, I consider a misnomer because it omits part numbers for parts considered obsolete.