I have a tub shower combo with a knob on the tub spigot to pull up to send the water to the shower head. In the shower position very little water comes out of the shower head. Most of the water comes out of the tub spigot. It is an old setup, dating from the 1960's so it may be completely worn out. What is the easiest fix for this? Can I just replace the spigot? Is it sold by itself? I would like to avoid breaking out the tile and changing all the plumbing right now.
Yes you should be able to just replace the spigot. You may also want to check to shower head and make sure that it's not clogged at all. In most situations the pieces that wear out on these units are replaceable. It's not that often the the diverter assembly has to be replaced.
If at some point you do have to replace the valve and the tile is still in good condition, depending on what's on the other side of the wall you can often access and replace the valve from the back side. If the other side of the wall is drywall or plaster that will be much easier to fix than tile.