The oval offers power skating for adults as well as kids. Cannot speak for the adult classes but my boy took a week long power skating camp there at Christmas and it was very good.
phdhockey.ca, they have their own facility and many resources so may be able to point you in the right direction.
canadahockeyinfo.com has a long listing of organisations, though a lot of the links are broken.
I've had a hockey player friends in the past find adverts on figure skating boards for one on one power skating at a reasonable prices and were very happy with the results.
Certain arenas in Calgary, not owned by the city (ie: Centennial Arenas) offer power skating, but hard to find via net, you would have to ring and see if adults classes available.
canprohockey.com is another good site with programs running in August.
The Source for Sports has knowledgeable skate sharpening staff. Take your skates in and ask them.
Make friends with someone who has a membership to the Glencoe or Winter Club.
Good luck
Power skating in Calgary
