I don't think they're discontinued; only that the USPS website is messed up. You can still order them via USPS
ebay (where several are noted to have been sold already). Here's the USPS
ebay link for the "without die cut" variety:
http://www.ebay.com/itm/USPS-New-Lo...em35d0f7876fAlso, I'm sure you could call the USPS at 1-800-STAMP-24 and order that way. (Personally, I have never ordered a thing from the USPS website, as it contains just too many errors or incomplete information.)