Whilst most people just connected their Commodore 64 up to a TV, others liked having a proper 'Commodore' Monitor.
The advantage to having a dedicated monitor for your Commodore 64 were many. Firstly, you didn't have to share the TV with anyone else. You could leave your C64 hooked up on a desk, connected to a monitor, without needing to move it.
Secondly, the image quality from a monitor was far better then the picture coming via the RF connector on the back of the C64. The Commodore Monitors brought Richer colours, static free picture and better image detail.
These days, C64 monitors are kind of rare. The Commodore 1702 is most common, however some people use a Commodore 1084 Amiga Monitor, which is a equal, if not better alternative. You'll still find them now and then at a local garagesale, but most of them have been snapped up by Commodore Collectors. However, there's usually a few available - Check out the listings below.
VINTAGE COMMODORE COLOR MONITOR MODEL 17...
COMMODORE 1080 AMIGA MONITOR IN EXCELLEN...
VINTAGE COMMODORE 64 MODEL 1702 COMPUTER...
VINTAGE COMMODORE 64 1702 MONITOR
COMMODORE AMIGA 1200 + COMMODORE 1084S M...
COMMODORE 1702 MONITOR
15KHZ LCD MONITOR 20 COMPATIBLE W COMMOD...
VINTAGE COMMODORE 1902A COLOR MONITOR