You can buy the MGMAT online, computer adaptive test bank for $39 from manhattangmat.com. They give you complete explanations for all of the questions from each test. You can take at least 6 tests without the questions being repeated, too.
I think that the old paper GMAT's are good for practice and timing, but taking a GMAT CAT would be more closely representative of the real thing. I would still take the other GMATPrep test from mba.com. Although those two GMATPrep tests don't come with any explanations, they are still good tests to take. You might have to dig deeper to find how they got the answers, but that is also a part of the learning process, too. If you have access to several editions of the Official Guide to the GMAT, there could be the chance that you will find the answers to some of the questions and problems in the GMATPrep tests.
Greg