I'm a Canon shooter, so you'll want to back up what I say with a little research.
Nikon's tubes are probably the most expensive (each of the three is about $85).
Kenko makes a set with contacts ($170-$180). But the best price I've seen is probably for the
Pro Optic set, at Adorama ($90 for the set). That listing looks a little weird--you might want to call and find out if it's the set with the metal mounts or the set with the plastic mounts (on the Canon side, the $90 set has metal mounts, the $60 set is called "budget" has plastic mounts).
You also need to be aware that when you use extension tubes, you lose the ability to focus to infinity. Typically, depending on how much extension you add, the combination will only focus at a given distance from the subject, so you won't have much choice on framing. This is why a macro lens is still worth the money.