The chrome letters on sides of bike I bought at Autozone or Advanced Auto Parts store, not sure which one, I have both in my town. They were stock items displayed on the rack right next to license frames. About $1 a piece.
The black letters on front of bike as well smaller chrome letters on the back of bike I ordered from internet: www.chromecarletters.com
Black ones are 1" Mercedes Benz Gunblack Style, while rear chrome ones are 3/4" Basic Style Small. You purchase them each, inexpensive, and they all have double sided tape on the back, just like the OEM ones.