Best place to look for these games is probably Half.com, I've found. The fourth part of the first series is especially hard to find, unless you don't mind paying at least $35-40 for it. Half.com has a number of copies of Infection for under $10, Mutation and Outbreak for under $20, but the cheapest copy of Quarantine is going for $44.84. None of the cheaper ones are sealed copies, though.