Google is probably your best bet. Federal should be easy. State and city/county will probably be harder. Most places have an issue weapon and then a list of allowed carry weapons. We are issued Sig 229's but can carry 226's or 227's and a number of backup guns.
Better to search by gun or wiki certain departments. Never seen a comprehensive list. There's a list floating around for the XD but some of the agencies no longer issue it. Also many departments allow a wide range of handguns which can be individually purchased and carried by their Officers.
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