Here's what to do.
Take a piece of tinfoil and wrap it around the avocado. Then, place the foily fruit on a baking sheet and throw it into the oven at 200 degrees for 10 minutes.
Avocados ripen from the ethylene gas that it produces. Because it's covered in the tin foil and heated, the gas surrounds the avocado and speeds up the ripening process exponentially.
It may take a little longer depending on how ripe your avocado still is, so do your best to limit the cooking time while you're doing the grocery shopping.
It'll at least save you the trip to another store.