I collect US banknotes, but at an even greater snail's pace than I do my Canadian notes. For the most part, I got mine from banks or in circulation, and most of mine are the green-sealed Federal Reserve Notes. I have the small headed ones ($1-$100), the large headed ones ($5-50), and the new coloured $20--looking forward to adding the coloured $50. Over the years, I have managed to obtain a couple of blue-sealed Silver Certificates, and one red-sealed United States Note, but that's it so far. And of course, I have my $2001-note and my $1,000,000-note.
For the most part, I just collect what I come across, as my banknote collection is in fact a spin-off of my coin collection.