Sent by God, yes. Sent by a church, No, never a requirement.
It seems that not just one or two people, but most American church leadership seem to think that a missionary should be sent or supported by a church and they call it biblical. I fail to see anywhere in the Bible that this is the case, unless one take verses out of context.
Yes, a pastor, preacher or a missionary bases his financial support chiefly on the church, but never is a church's permission required. That is how new churches or new denominations get started. Therefore, when someone introduced a missionary to me, I never care about what church sent him first. I look at his relationship with God and character first. I could care less about the church that send him because if I have to consider the church, I would have to consider what problem that church has as well and how it affects or not affects him.
So in my own view, I tend to deduct value points on those who rely heavily on the churches /ministries that sent them, and then judge them according to their religious value later.
If a missionary does not rely on a church, but there's a church that claims otherwise, this church already has big big problem, but the missionary's value is yet to be decided, regardless of his need to publicly disassociate himself from the church. Although, what opportunity he has to deal with such church should be looked at closely, namely, how is he rebuking such church.
I also despise the view that a missionary must or better to have some common careers alongside their ministry. Like some group who went to China as doctors, but claiming to churches that they are missionaries. To that I can simply reply: Oh, you are missionary? So am I and all my church members!
Being a missionary has this requirement: Willing to risk one's own life for the Kingdom of God. Having other job security and whatever else kind of insurance are exactly the opposite of it.