Here's a very comprehensive list of adverbs of frequency for your ELT classes. It's a set of worksheets which includes an activity, a complete list as a reference point and a suggested answer key.
In the activity, you can instruct students to work in pairs and try to complete the list according to their frequency. There are many grammar references which indicate all the adverbs with a specific percent. I don't think it's a proper way to classify the adverbs according to particular cents, so I've just pointed always and never as %100 and %0 and left the rest empty. All the adverbs are listed in a scale from %100 to %0 relative to their frequencies, which I believe as a better way of classifying them. However, I also have left the percent boxes empty for you to clarify the frequencies of the adverbs further if you want.
There's also the complete version of this worksheet. You can give it as a grammar guide, and your students may use it as a reference point in their studies.
The answer key is called suggested answer key because of the ambiguous meanings according to different people when placing these adverbs according to their frequencies. Also, there are some more adverbs given in the answer key if you like to add other common adverbs to the list.