You are indeed correct that such an expression is not found in the Scripture. The idea of being "slain in the Spirit" comes from an attempt to explain the ecstatic experience of losing control of oneself and being possessed by some supernatural power. However, the excesses that are endorsed under the description "slain in the spirit" is grossly contradictory to what the Scripture teaches is the impact of the Holy spirit's presence and work in one's life. According to Galatians 5:22-23, the fruit of the Spirit produces "... self-control."
Convulsions, twitching, eyes rolling around, shaking, laughing, ecstatic utterances, and the like are all inconsistent with the decorum associated with the self-control produced by the presence of the Holy Spirit. These excessive indications are more consistent with the descriptions of those demon possessed throughout the New Testament.
In short, being "slain in the spirit" is not a biblical concept.