I would say it depends on the club. If it is a driver/wood with a sure fit adaptor or similar it would be quite easy but finding a single iron head might be more difficult. What club is it?
No idea on the cost of a single for those. You could ask a local pro or golf shop to order you one. Sometimes single shafts come up on E-Bay if you keep an eye on there.
Far too many variables for me to take this job on. Yes as stated they do come up on e bay, but will it be the right flex etc? so you could be in for a wait. I would take it to my Pro, if they couldn't do the job they will send it to Mizuno. It will not be as pricey as you think.