A water pump bearing is pressed into its housing. And sometimes we inject nylon or use a snap ring to make sure it does'nt move. Even if you could remove and install a new bearing w/o brinelling it.. you as joe consumer can't buy the bearing you need. About your only choices is to buy a used one or a "new" one. (sometimes re-man)
Also, a word to the wise in aftermarket water pumps. Buy an OEM replacement aka "service part". Even if it costs more. Go this route and save yourself a lot of potential headaches.
btw, I was a WP/FEAD engineer HERE
for 6 years. (Back then we were the worlds largest WP manufacturer. Both OEM and aftermarket)