. Where can I buy a fin for this board?

Won't happen. Steer your kayak with paddles or learn to do that.

Look inside the drum at the place where the fin was on the drum.
Do you see any screw holes now showing?
That will tell you that you need to take the machine all apart and re-fasten the fin on with some screws, unless the holes are stripped out in the fin itself, in which you will need to get a little bigger screws, or buy a new fin.
Oh yes, I have been using an epoxy cement in a tube that has 2 chambers, and this may help you.
I think it would work here, but will not be strong enough to glue the fin back on.
I sort of think it would stand the heat if you used it to help secure the screws into the fin.
Honestly, to me the new fin sounds " hard to get".
Be blessed.

The fins can be easily removed to transporting the paddle board, however they are used for navigation of the water so you should not try to paddle board without the fins attached.

Most paddle boards are made with traction pads attached to the top of the board, these should never be waxed. Boards that do not have the traction pads should be lightly covered with a base wax.

The elbow in a paddle provides a more powerful, effective stroke. When you're paddling, the elbow causes the paddle blade to align straight up and down as it comes alongside the paddleboard.

Taken from: http://www.rei.com/expertadvice/articles/paddleboarding.html

That depends on two factors. You have to determine if you want a different type of board for each different type of wave so that you can cater the board to the wave. Also, if you are travelling a lot to different surf destinations, I suggest getter a board with removable fins because it is less likely to get damages under the plane and you can store the extra fins in your luggage.

its just dirty contacts if you can open your keyboard, clean contacts with non rubber eraser like the ones used by engineers and draftsmen. also include the rubber contacts under the keys

Ask the manufacturer at:
http://www.freedomhawkkayaks.com/contact.php