hmm bro, honestly if you're looking at 2nd hand tanks, the dimension u're looking for won't be common like wht bro terryz said, moreover, adding to bro lemon's point... be prepared to submerge your upper body underwater if you need to do maintenance at the very bottom of the tank (ie. your coral frag which you place on top drop to the bottom of the tank, or trying to get a pest or fish out).
More common cube tanks are 2ft.. u can search in the pasar malam thread for it.. else try typing in key words like WTS Tank to search for it, hope it helps 1 more thing.. might wanna read up a bit bout getting 2nd hand tanks here : @ http://www.sgreefclub.com/forum/index.php?showtopic=90210 and make sure that the cost of replacing and repairing parts of a tank does not cost too much, else might be better off getting a new tank.
just my 2 cents worth.