What is being "slain in the Spirit" and where in the Bible can it be found?
Being "slain in the Spirit" refers to the act when people go before a Minister or Pastor or another individual, where they lay hands on the person, for the purpose of being healed of some affliction. The person then falls back or forward sometimes shaking ,as that makes them and everyone watching believe they are being exposed to the Holy Spirit.
There are several verses in the Bible that these people use to justify such actions, such as Revelation 1:17, and Daniel 10:7-9 and John 18:4-6 to name a few. However, the interpretation and context of these scriptures speak about times when God spoke to the people through visions, and they were overwhelmed with the presence of those visions. In the verses of John 18:4-6, these men 'fell to the ground' when Jesus revealed His deity. Knowing who He was was the overwhelming reason for them to fall to the ground. Could it have been the Holy Spirit convicting them? Possibly.
However, the situation as noted in the first paragraph that people get involved with today, in my opinion has no biblical support. Unfortunately, these 'acts' have also been proven to be staged and sometimes the participants are even paid by the Evangelical Preacher, or whomever, for the glory of themselves.
Having contact with the Holy Spirit is something that does in fact happen when the 'baptism in the Holy Spirit' occurs;
"I indeed baptized you with water, but He will baptize you with the Holy Spirit." Mark 1:8
This baptism of the Holy Spirit does not occur by man's "stage act", but by the calling of god when His grace of salvation is bestowed on the person.