Hi
I know you say primary documents only and I'm not sure if this will count, but there could be quite a lot of published letters/extracts in the papers from that period.
e.g.
http://www.britishnewspaperarchive.co.uk "... THE INDIAN MUTINY. The following is extracted from the private letter of a young officer in India: "” ... "
Sat 08 Aug 1857, London Standard, London, England
"... THE INDIAN MUTINY. The following extracts are made from a letter received by the family of a young man ... "
Thu 05 Nov 1857, London Standard, London, England
"... THE INDIAN MUTINY. LETTER FROM A NATIVE OF HUDDERSFIELD. The following ... "
Sat 15 Aug 1857, Huddersfield Chronicle, West Yorkshire, England
If you could use that sort of material then you could search first and just download like mad to keep the cost to a minimum.