How can I find the exact written versions of commonly spoken Cantonese words?
There are a lot of Cantonese dictionaries online. The one I often use is CantoDict. It is volunteer-contributed and is pretty comprehensive. I has both characters and words, and has pronunciation for both Cantonese in Jyutping and Mandarin in pinyin, meanings in English. and you can search based on characters, Jyutping, pinyin, or English.

Once you get the hang of Jyutping, that will be the easiest way to search for something you know how to pronounce in Cantonese but don't know the character for.