I used to search it on Excel Add-in but I do not have it on my machine anymore.
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I have used this online tool to search for instruments and data types for Datastream.
DSWS API - What is the pricing for this method of accessing the data?
Datastream definitional notes
How Expressions are accounted in DSWS request limits?
Can you easily convert a DSDataResponse object to a datatable in C#
DSWS API - SEARCH FOR COMPANIES AND CODES