You will find mussels and oysters any month on PEI however, luckily for you October is one of the better months as the water is cooling down and the mussels and oysters plump up a bit!
You will find both in most restaurants notably- Sims Corner Steakhouse and Oyster Bar, Claddagh Oyster House, Merchantmen Pub and Carr’s Oyster Bar! Also available in any fishmart or grocery store if you want to cook and shuck your own!
Lots of cool info on mussels at https://peimussel.com/ and some more info on our thriving oyster industry https://www.tourismpei.com/oysters-recipe.
Hope you enjoy your stay on PEI and it is an awesome adventure trying all the different varieties of oysters and tasting the difference, here on PEI - you will find no fresher!