Advice please Transom Gasket
 

I have a split in the gasket (boot?) where the outdrive of my OMC leaves
the hull. I need

a) a good source for gaskets

b) Advice on what to watch out for when changing

c) Perhaps advice NOT to try it myself, and leave it to a pro

(if this is the case)

d) What should I expect to pay?

Thank you gentlemen for anything you might offer