The difficult part can be getting the old finger buttons out on some of the older models where they are held in place by shrinking the cup around the outside of the pearl. Some keys have a hole beneath the pearl that allows the pearl to be tapped out. In most other cases the pearl can be pried out using a large needle under the edge.
There are lots of pearl sets for sale on the internet. They are not the same size on all makes and models so be sure to measure first---usually in millimeters. The real mother of pearl ones are more expensive. The plastic ones dubbed "mother of toilet seat" are cheaper. I install them using gap filling super glue. It is important not to use too much glue or when you push the pearl firmly into the cup it can come out the sides and glue your finger to the key. Don't ask me how I know this.