Unlike OP as in below thread, I don't have any problem using my Canadian credit cards online.
However, I realized that I spend around $5,000 a year in USA. Because I live in Yaletown, I always exchange money at one of exchange houses in downtown for good exchange rate of 0.5-1% comission over official bank rate, I've been carrying large amount of cash on each trips.
I thought it'd be nice to earn rewards on my spendings in USA, and I don't want to carry large amount of cash all the time.
I tried to apply credit cards with many different companies online, but all of them require to me being a resident of USA and a social security number.
Some people suggested me to get a Canadian US Dollar credit cards, but I learned that they charge around 2.5% when converting from C$ to US$ to make payments to my account, so certainly not as economical as carrying cash.
Let me know if you already have US cash back credit cards, and how you got them.