This is an excellent question. I have a Dansco with every Kennedy through 2006 and missing only 2007 P & D. I am wondering if I should buy the next page and start filling 2008 and beyond (I will for sure fill 2007 P&D to fill those two holes in my current album). I never see any in circulation and I asked my bank for some, and they didn't have ANY...
2001 was the last years Kennedy Halves were made for banks.Only way to get them is at the www.usmint.gov or coin shops or Ebay. I strongly recommend they be bought.The 2008 has a lower Mintage than some Walking Liberty or Franklin half dollars.
Kennedy Halves will be popular with the public again if they reduce the size of the coin. Then they could mint more of them. One idea is to make the coin with multiple sides. I only have Kennedy halves through 2001 with some missing. I check rolls of them from the bank. Coins from 1999 on are almost as hard to find as silver half dollars!