Indeed, it's now impossible to find Quatro in stores; the whole line appears to have lasted only from 2004 to '06. I recently wanted to find some Quatro sets to introduce my new niece to LEGO (got to start kids off right
;) and was agog to discover not only could I no longer find Quatro sets in LEGO's online store, but people were offering them secondhand online, and asking outrageous prices, like multiple hundreds of dollars for tubs of a few dozen bricks. I had no idea the line had been discontinued so quickly, and no idea people were asking the same sorts of prices for these they'd ask for discontinued
Star Wars sets, sets from the old Classic Space and Castle lines, etc.
Duplo sets are still easy to find, though, in my experience. That line's been around a lot longer than Quatro, too, and doesn't seem to be slowing down. I think Duplo will be around a long time to come.