We had a fun time matching these opposite cards yesterday! M. grasped the concept pretty quickly and was easily able to match some of the more familiar concepts (day/night, big/small, boy/girl, etc.) and after a few times through with me returned to match several times during the course of the day.
The opposite cards are from Montessori for Everyone. They have quite a collection of printable Montessori materials across the curriculum. If you have a little extra in the budget, I recommend them. It certainly saves time trying to track down images all over the web (which may be copyrighted!) and they will send printed files or PDF via email. A great resource!