If you are looking specifically for "Fatboy" speakers you get them Lionel. Go to lionelsupport.com and under Replacement Parts use the pull down menu and select part name rather than product number or name and type in "fatboy". There are basically four choices. 8 ohm and 18 ohm 50 mm "Fatboys" and 8 and 16 ohm 40 mm "Baby fatboys". What you select will depend if you have room for one or two speakers and how large. If two speakers you would want two 16 ohm speakers. If one you would want 8 ohm speakers.
Pete