Depending on your motor, Napa carries them. Probably have to be a common motor for them to have it in stock unless your very close to a large lake where there is more need.
O'Riellys also carries sierra replacement parts.
Also degree of dificulty depends on the motor but I find most are very easy to replace.
Aside from that, youll have to get them from a marina if they are open. You can order them online as well and ahve them shipped fast.