Just be sure to get one of the sets with the electronic contacts.... unless you only use vintage lenses with manually controlled apertures. With most electronically controlled lenses, it's a real pain in the arse to use the super cheap ext. tubes (under $25).
That means about $50 minimum, for a rather plasticky set, including the bayonet mount (Zeikos). For around $75 you can get plasticky with a metal bayonet mount (Opteka, Zeikos).
The Kenko set is still better... largely metal and about $200 now... that's roughly equal in quality to but works to be a bit less expensive than the Canon tubes, which are only sold individually.
Whatever you choose, if you want to use them with EF-S lenses be sure to check that they are compatible. Most currently offered are... but if you buy used, for example, you will find some older ones can't fit EF-S lenses (note: EF and third party crop-only lenses will work fine).