Camping and outdoor gear stores sometimes have them. Very convenient on the trail, or in ski boots, when the temperature changes, or it starts to rain. Most are just thin nylon rain pants, but there are others.
For example, search for
But some camping and outdoor wear is more practical than stylish.
Most skate pro shops are small, so clothing selection is limited. You would be better off going to a big department store, or a big sports gear store (like Sports Authority, where I've seen them). If there is a big discount clothing store, they would be a good place to look cheap.
I keep them in my pack when I hike and ski, to handle potential weather issues. A ski instructor said she would be impressed if I could put them on without removing my skis. I had trouble figuring out what snaps and zips went together where. Best to practice in the house, when no one is around to laugh at your mistakes.