I have some spare Slingshot fins I can hook you up with. What year is your board and what fins do you have (in 2013 they switched from the 8" to 6" long fin on the recoil). I have some of both I would share.....
When I get home tonight (if I ever get home..... on second delay from Orlando-Kansas City), I'll measure the hole spacing on my slingshots and let you know, that should tell you if the holes line up. Next obstacle is screw length, since the slingshot boards are so thin you may find the covin screws bottom out in the fins before getting tight. Hardware store will have shorter screws and way cheaper than new slingshot fins.
Like I said, let me know what fin size you need and I'll mail you one, how did you break a fin anyways?