How to find distance from center of object to the lowest point of object

I have FBX model and I want to find how high it should be (position) to its lowest face is on ground (high 0).

I tried renderer.bounds, collider.bounds, mesh.bounds but size of anything is 0,0,0.

This is how I create object:

GameObject obj = Instantiate(Resources.Load("I",typeof(GameObject))) as GameObject;
obj.name="kocka123";

Get the mesh of your object and iterate through all of them to find which has the lowest y value.

    Mesh mesh = GetComponent<MeshFilter>().mesh;
    Vector3[] vertices = mesh.vertices;
    float lowest = Mathf.Infinity;
    int i = 0;
    while (i < vertices.Length) {
        if(vertices_.y < lowest) lowest = vertices*.y;*_

i++;
}

Looking at the meshfilter works, but you should also use transform.point to on the vertices to ensure you get their world-space position.

Using Linq you can do this:
float minY = transform.GetComponentsInChildren().SelectMany(m => m.mesh.vertices).Min(v => transform.TransformPoint(v).y);

It iterates over all meshfilters in all children, selects all their vertices and finds the lowest/min y point.