The Vello set for $79.95, found from the above link from gryphonslair99, looked to me like a decent metal tube alternative to the Kenko set., which is now around $124 or so--about where it has been priced for the last decade. Keep in mind--there are now a lot of plastic extension tube sets, and some brands have a choice of plastic at $49, or metal at around $65 or so.
Some vendors used to sell extension tubes as single units: the 12 and the 20mm are THE most-useful with most real-world lenses like 70-200 or 85mm etc; the two paired make 32mm extension. The long, 36mm tube is one that just is not all that uself for me--it's just too doggone much extension most of the time.
Some lenses will not function with extension tubes that are very long: short focal length lenses can quite easily have the MFD moved up to within the inside of the lens itself! This is especially common with like a 12mm tube on a 24-70mm lens; only the longer focal length range of the lens will actually allow you to focus on an object. For this reason, Nikon used to make some ultra-short K-ring sets, with very, very small amounts of extension, like as I recall 5.8mm as the very-longest of the five-ring set. These were for lenses from 28mm to 200mm if my memory is right.
It is likely that all you'll really need is a 12 or 13mm ring, and a 20mm ring. Some used to have 13mm, other brands had 12mm as the "short" tube.