Hi There
I dunno how recently it was Trikky did this but I did this just a month ago. Well, from Portland to LA anyway, so I cannot comment on Canada.
I tried the over the counter thing at 2 or 3 places and was turned down flat!
Here is what I found out........
To do the job properly, you need to register the vehicle in your name. To do this you need a US address. Then you can get insurance.
I didn't have an address but luckily a mate on the East Coast was able to put it on his insurance just using the VIN so we didn´t need to register it in either my or his name, and I just paid him the extra premium. Insurance in the US works different from the UK. Its the car that is insured, not the driver so this was the easiest way of doing it. I got a letter from my mate along with the cover note, saying we had his permission to drive.
Getting Canadian insurance to cover you in the US might be easier for us Brits. I dunno, but I'd try there first
Do a google search for non resident car insurance in the USA
Don´t be put off though, Its a bloody fantastic trip.
Let me know if you need any other help/advice