To obtain an old amber from FR,LG,Red,Blue you must go to the back door of the museum in Pewter City by using cut.
Talk to one of the scientist and he would give you Old amber.
In HG/SS go to where union cave is at next to Violet City and rock smash all the rocks you see. If you didn't find any exit the area and return and repeat the smashing!
You may also find Helix and Dome fossils as well!