What are some good quotes that show Lennie and George have talked about the 'dream farm' many times before?
We actually see quite a few references to this. Lennie clearly has troubles with his memory, yet after George gets angry and tries to console Lennie, Lennie quickly demands, "Tell me--like you done before." For Lennie to remember the farm and not the girl in Weed or the fact that George has his workcard means that George has repeated the story.
When George does start telling the story, "Lennie broke in," which means that Lennie recognizes the story. The next lines are in italics, suggesting that Lennie is repeating them word for word from some original. "But not us! An' Why? Because... because I got you to look after me, and you got me to look after you, and that's why."
In the last chapter, you see a repetition of these words that is so close that it's almost as if these two have memorized the words.
Lennie (prompting George): "Because--"
George: "Because I got you an' --"
"An' I got you. We got each other, that's what."
So clearly, this story has become something of a personal mythology or mantra for them.