Oh, dear, E, I forgot that "ain't" for some reason changes the verb around it (at least sometimes). No wonder you're confused. You probably saw 'ain't got it', not ain't get it. "Ain't got it" = don't have it*, as in, "I ain't got that book you've been looking for" (though a person who uses the word ain't would probably say 'you been looking for, leaving off the 'have'). We would not say 'ain't have it', and I have no idea why.

*Unless they mean a colloquial meaning of "getting it": understanding. If someone explains a joke to me, then I "get it". If they explained it to me, then I "got it", meaning I understood it. We don't use the 'have' with this sense of 'get'. But there is yet another possibility, that I guess has to come from context. I could say, "I got the joke" and mean that I understood why it was funny. I could also say, "I got the joke" and mean that I, for example, got a friend to write down the joke that you had asked for--literally have it in my possession, that is. A further intricacy of this is that if I picked up a copy of the joke, it was in the past. I could say, "I got the joke", meaning "I picked it up an hour ago", or "I've got the joke", meaning, "Here it is, in my possession right now". Get it? Oh, if she does, people, it will be because of her extraordinary understanding! If anyone can make this muddle clearer, I wish you would.