Look for the job now. While you're still employed, you have an airtight excuse for not providing a manager reference from your current job ("They don't know I'm looking."). That lets you provide either your last manager, or a sympathetic coworker instead.
Wait until you're gone and it gets much sketchier. People will assume the worst if you don't provide your previous manager.
Wait until you're gone and it gets much sketchier. People will assume the worst if you don't provide your previous manager.