I don't have a specific recommendation because I don't use any of them, but as far as I understand, prediction markets allow users to propose new predictions that can be bet on. Another option is to simply attempt to directly bet someone who is willing to take the opposite bet. There should be no shortage of takers. This is obviously riskier since you're not a book and can't do much to enforce payment, so usually a very public bet with someone who is well-funded and would rather pay than suffer the embarrassment of backing out is a typical approach.