I am guessing that ATF would be the best (most reliable) source for that sort of listing. But such a list would not be all-inclusive since, believe or not, some agencies allow their officers to carry whatever they choose so long its within approved caliber ranges. Crazy, right?
Gaston figured it out a long time ago. When he began thinking about marketing his 'new' plastic pistol, he asked himself 'would I rather have 1% of 10 million or 25% of 100 thousand'. The answer was simple, sell pistols so cheap, no other firearms manufacture can compete. Sell at or below cost....making every LE agency an offer they can't refuse. Name recognition and cheap pricing. The rest is history.