The Highland Archives in Inverness has some records of the Ross-Shire Militia listed as general and garrison order book 1813-1814 see
http://discovery.nationalarchives.gov.uk/details/rd/N1388033... for contact information.
Some are also held at the National Archives of Scotland (NAS) in Edinburgh
see their research guide at
http://discovery.nationalarchives.gov.uk/details/rd/N1388033... . Although the catalogue references may have changed their Ross-shire Militia holdings are listed on page 45 of “Militia Lists and Musters, 1757-1876: A Directory of Holdings in the British Isles”, available online at
https://books.google.co.uk/books?id=K2h6b2T0yJUC&printse... The National Archives, Kew also has some Muster Lists & Pay Lists in document series WO13 for Ross, Ross (North British) & the Rossshire (sic) Ross Volunteers and Militia covering the 1805 period; to see the catalogued list from
http://discovery.nationalarchives.gov.uk/ > advance search (yellow text) enter Ross; Ref WO 13; dates 1803 – 1808
None are online.
Unfortunately attestation papers of militia in document series WO96 are only preserved from the mid 1800s.
From 1786 the full title of the 72nd Highlanders was the 72nd (Highland) Regiment of Foot; in 1809 the “Highland” was removed from its title and from Dec 1823 it was renamed the 72nd (or Duke of Albany’s Own Highlanders) Regiment of Foot.
After reading below, in the first instance for the 72nd Highlanders you may wish to contact the Highlanders Museum at Fort George to see what records they may hold, see
http://www.thehighlandersmuseum.com/ (if the site’s tabs don’t work Google for Highlanders Museum and click on sub-links)
Records for regular army soldiers discharged to pension are held in document series WO 97 at the National Archives, Kew and online at findmypast; there doesn’t appear to be record for Donald Bain or Baine who served in the 72nd Highlanders so he wasn’t discharged to pension.
Prior to 1883, records of regular soldiers were only preserved if they were discharged to pension, the only information for men discharged for any other reason, including death in service, will only be found in Muster Lists or Description Books.
If the Highlanders Museum doesn’t have any, although not online the National Archives, Kew has muster books of the 72nd Regiment of Foot covering c1800 to c1850 for 1st battalion & c1804 to c1816 for 2nd battalion in document series WO12 (use discovery link above to search); I couldn’t identify any description books.
Good hunting.