So it appears that the USPS no longer sells anything less than full sheets of stamps online, except for the Priority Mail $7 and $29 stamps, which I can still purchase as singles. (The $5 Liberty stamp is only sold as a sheet of 4.) I thought maybe this was a web glitch but the USA Philatelic catalog only lists sheets as well.
As if collecting new issues wasn't pricey enough...
I believe that you can still special order blocks of non-s/s issues for a special handling fee. They stopped packaging blocks about a year ago (with a couple of exceptions). The site does not explain this option, but it is explained in their philatelic catalog as to procedure & fee.
Virtual Stamp Club shows order numbers for 4 stamp blocks on their page. Doing a search for one of those numbers on USPS's websites brings up nothing. So I guess it's true? That's disappointing. I stopped today at a local post office and picked up George Bush and USS Missouri stamps. They sold me a block of Bush, but I had to have a whole sheet of the Missouri stamps. It's awfully pretty.