The best examples of dialect in Of Mice and Men are in the dialogue of Crooks and Candy. Chapter 4 is full of dialect by Crooks while he is talking to Lennie in his room and later to some of the others. Here is an example:
"I said s'pose Geoge went into town tonight and you never heard of him no more. . . . Nobody can't tell what a guy'll do. . . . Le's say he wants to come back and can't. S'pose he gets killed or hurt so he can't come back." (p. 71)
A good example of Candy's dialect can be found in his speech to the dead girl in the barn in Chapter 5.
"You God damn tramp," he said viciously. "You done it, di'n't you? I s'pose you're glad. Ever'body knowed you'd mess things up. You wasn't no good. You ain't no good now, you lousy tart." (p. 95)
Dialect used to be very popular in American literature. Mark Twain's Huckleberry Finn is full of it. American humorists like Artemis Ward used dialect heavily for evoking laughter. But readers became more sophisticated. By Steinbeck's time dialect was mainly suggested rather than literally transcribed.
Dialect refers to the way people from different areas of the country speak. They each have their own sayings that are common to a specific place and time in history. If you read somethiing and it sounds strange to you that is probably due to the dialect someone is speaking in. Look for old fashioned statements that would have been spoken during the time period the story is written in and remember that Of Mice and Men takes place during the Great Depression. People had to travel to find work so many people had country accents, even in California. Many of the characters lacked formal education so that may help you find those quotes as well.