I dont think you can replace just the bearing.. but can replace a fan..... I would replace them both so I wouldnt have to worry about the other one going out in a month or three....
As a side note, I had my fan assembly out and when I put it back in I forgot to plug it back in. I drove around for days like that and had no heating issues. My temp needle was only slightly above the normal position. I was very surprised at how cool it stayed without the fan running. I do not recommend this practice!