Yes, there are some still existing handwritten documents of Johann Sebastian Bach preserved. There are very few letters of him and there are - see me smiling - Bach's famous petitions and complaints, which are conserved and stored carefully in the Bach cities of Weimar, Arnstadt and Leipzig. In addition there are relatively many receipts, which Bach had written by hand.
Okay, the term "many" is slightly extreme. However, there are so many, that there are books, which collect them all and allow later generations to have them accessible to enjoy them.