Plea agreements, yes, but agreeing to cooperate is generally only available via "discovery" records from the specific court cases the informants worked on. In other words, "discovery" includes the details of how the state obtained their evidence, and generally includes the name of the informant. Confidential informant(CI) lists have been known to make their way into circulation among the criminal underworld from time to time, as well, but it isn't legal(or ethical, obviously) for LE to let those things slip. It does, however, happen.
ETA: Sometimes informants also agree to get info(texts, etc) on their cell phones as a "safer" means of informing. In that case, there is usually public record of a warrant for the subject's call phone. It's not a "guarantee" that they willingly informed, but in some cases, folks seem to get branded as informants if there is any suspicion at all. The drug world, and especially wrt meth, is highly paranoid. JMO