I was playing with the KimUno simulator for the old KIM-1 microcomputer, and started playing with the source code a bit, and ended up deciding that I'd really like to have the source code for the monitor itself. A little bit of digging found a variety of sources for the ROM: I found ROM dumps and some slightly mangled source here and perhaps a more authoritative source from Commodore here.
The source code from the first link was in a slightly bad state in terms of formatting and syntax. I wanted to use the assembler from cc65 and so the code needed a bit of cleanup. I spent an hour realigning the source code, fixing comments, and adding a config file for ld65. If you have cc65, you can recompile the source using the commands in the file "assembleit". You can compare the hexdumps of the original roms with the output to make sure it's compiled the code properly.