While I have no specific info for MF ex USA, most mainland airlines that I know offer them at the check-in desk for international routes and in the app for domestic. The price for long-haul international routes tends to be between 3000 and 6000 CNY. Again, most airlines also note that the upgrade only includes the seat, not the lounge and (more importantly) no J cl catering.

You may want to call their airport office (not city sales office) to double check.