To the IPA, FBI, and RCMP, I'll add:
US Border Patrol
New York State Police
New York State Troopers
Connecticut State Police
Michigan State Police
California Highway Patrol

I just did a search for "police" at the Kilt Store, and 20 matches (some duplicates to the above) came up. There are tartans out there.

As for an universal LEO tartan, I can't see it happening. It's a nice idea, but some of us are so territorial about our jurisdictions that the idea of all of us wearing a single tartan would be rejected out of hand.
Or to put it a different way, I can't picture a ceremonial occasion where a US Customs officer, a NYS Trooper, and Lewiston NY PD all wore half the same uniform. Border Patrol would be standing off to the side in his tartan laughing at us.