I think you might mean "file a lawsuit", which means to initiate legal action, to initiate a civil suit, to use the court system to try to get money from someone because of injury or damage

This is not an idiomatic verb and noun collation. But that doesn't necessarily mean it has no meaning. It depends on the context. If this is definitely what you read/heard, and it was produced by a native speaker or in a published text, you would need to provide the entire context. The speaker might be making a creative/humorous use of language to make a point. For example, whereas, yes, we normally say "file a lawsuit," someone might "buy a lawsuit" by bribing the judge so that they win (thus, "buying the (outcome of the) lawsuit". Again, though, this would be a unique and non-idiomatic phrase and we would need to see the whole context.

"buy a lawsuit" has no meaning to me.