Buying the cutless bearing is the easy part...
...Replacing it is another story. You can buy the cutless bearing from West Marine or Boat US. You need to know the shaft diameter, bearing length and OD. But if you are new this, I srongly suggest you have the yard replace it. If you really insist on doing it yourself, you can find lots of discussion on the subject and drawings of tool you have to make in order to do the job right. The bearing has to be pressed out and new one pressed in. You also will need a prop puller. Keep us posted on how you do.Lee