Hello Hilary
The Reference Department of Central Library in Stirling has a few trade directories for the early 1900s. It might be worth trying there. They also have the 'Stirling Journal and Advertiser' on microfilm; it's indexed from 1820, when it was started up, to 1970, when it closed down, so the staff might be able to trace any references to the firm. The email is centrallibrary[at][/blue]stirling.gov.uk (Replace [at] with @)
They also have the 'Stirling Observer' on microfilm from 1836 right up to date. It's not indexed, but any entry re the firm in the 'Advertiser' can be cross checked in the 'Observer'.
Hannah