I second Bikes Direct, I have 3 bikes from there (including a fatty) and am very happy with all of them. Framed also makes some nice affordable bikes, but from the shopping I've done are often a little more $ than comparable BD bikes. This can vary of course, so I recommend doing your own comparison shopping. Also check FB or Craigslist for used fatties, although depending on where you live the selection currently up for sale may not be all that great.
If you are looking for something to ride in deeper snow, don't go single-speed. You will need gears. Also, the fatter the tires, the better you will handle the snow. The Boris X5 on BD might be a good fit at $599.
Good luck!