You can get the expansion disc which is out in stores, it contains both those pieces of DLC. It's about a tenner here. Or, you can get Games for Windows Live (you'll need a passport account or w/e they're called now), which is like Microsoft's version of Steam (but incredibly dumbed down and with about five supported games), and download them through that. I would opt for the first choice.
There is another piece of DLC which is out, called Broken Steel (which is totally worth getting), and there two more pieces set to come out, Point Lookout (which is out in a couple weeks or so, and will be sold on a disc with Broken Steel and on it's own), and Mothership Zeta, which will sell on it's own too and as part of a bundle with the game and all five DLCs.