Well, it depends on what kind of sewing you do, how much you sew, what kind of machine you think would benefit you the most, and how much money you're willing to spend to get it.
Sears sells Kenmore. Wal Mart and Target sell Singer. For a Viking, Janome or Bernina, you'll have to go to an authorized dealer. If you're just wanting a machine that sews only, you can find an inexpensive decent one for a few hundred that will last a long time, but probably has no bells and whistles. The more money you pay, the more frills you get. If you're looking to embroider as well as sew, you're looking at around $1k for an entry level machine. Viking, Janome and Bernina are the top three manufacturers in the US, and they all make very good machines. If you sew a lot, I would not recommend a Singer or a Kenmore. That's just my personal opinion, though.
ETA: Corrected Bernina instead of Brother. I had a brain fart, sorry.