Criminal Law Questions? Ask a Criminal Lawyer.
Either could possibly testify against you. The fact that the confidential informant is in jail doesn't stop him from testifying. In fact, it makes it easier for the government to find him and get him to court. As for the DEA agent, that may be more difficult for them, but not impossible. They can try to subpoena him to come to court if he does not come willingly, as long as he was not fired for an integrity related incident that would make him lack credibility in court. If they can't get him, the prosecutor will have to make a decision based on what evidence is left as to whether they can go forward at this point or not.
If you haven't been sentenced yet then the court may allow you to withdraw your plea upon good reason. If the government would be unable to proceed against you, and you did not have that information at the time you entered your plea, then you may be allowed to withdraw it.
Before withdrawing it, however, I would discuss with your attorney what the prosecution's case looks like now. If you withdraw your plea and they can still go forward, then you lose the benefit of the bargain, so I would talk to him/her beforehand to make sure that the case against you isn't viable any more.