No idea - its a Peter Beales rose that first appeared in the UK in 2012, so you could email Peter Beales to see if they know if its available in the States and where. Seems like a job for Google...

There's a much older Red Letter Day bred early 20th century by the Dickson family. Something at the back of my mind says that American Pillar is a cross from that or it may be the other way round.

Interesting, I didn't know there'd been an earlier rose with that name, Urbanite. Not sure its available anywhere, and apparently it suffered terribly from blackspot, but there is a difference between the modern Red Letter Day and the old one - the modern one's a shrub rose, whereas the original was an HT. And maybe the fact that name's been assigned to a newer rose means the old one is no longer being sold.

Not sure which one of these you're trying to find Collovas, but the HT version doesn't seem to be available at all - this is what it looked like though

http://www.helpmefind.com/rose/l.php?l=2.19297.1