The books that I've read say that cam bearings almost never wear out. They certainly shouldn't be worn out after 50,000 miles.
If they've been damaged during camshaft removal/installation, take the engine to a pro for replacement.
edit: since you asked, for removal you need a special mandrel that fits the bearing ID, shoulders against the bearing shell and clears the bearing bore in the block. You'll also need a long bar to reach inside the block.
For installation: the bearings are pressed in. They must be pressed in squarely! Then they may need to be trued (there are various methods to do this, probably a boring bar is preferred). You can also blue and scrape the bearings. The books I've read say that it's a really bad idea to do this yourself.