You may want to go to a camera website such as B&H Photo or Adorama. They have a category for "rain covers".
I have a ThinkTank "Hydrophobia" cover designed for 70-200mm lenses... they make another for 300-600 zooms. But you also have to buy an eye-cup (you remove the Canon eye-cup and insert the special eye-cup that creates a seal for the rain-cover. ThinkTank's gear is on the pricey end.
On the low-cost end there are rain-covers which are so cheap they are more-or-less disposable (a pair of covers for less than $10). The cheap covers are made by OP/TECH.