I doubt that there would be many original sources online and free. Since this mutiny involved Indian (East India) rebellion again the British, the two place you might check are the National Archives of India website [http://nationalarchives.nic.in/ ] and the National Archives of Britain [https://www.nationalarchives.gov.uk/ ]
Found a book on googlebooks.com that can be read online
The Sepoy Revolt by Henry Mead 1858
A lot of information here
Dear Internet Person,
Thank you for posting your request on History Hub!
For the information you seek, we suggest you contact The National Archives of the United Kingdom, Kew, Richmond, Surrey, TW9 4DU, United Kingdom.
Since the mutiny or Indian revolt was against the British East India Company in India, please contact the British Library for access to their archives.
We hope this is helpful. Best of luck with your research!