private TreeNode FindNode(TreeView tv, TreeNode node, string searchString)
        {
            return tv.Nodes.Cast<TreeNode>().SelectMany(GetNodeBranch).ToArray().FirstOrDefault(p => p.Text.ToUpper().Contains(searchString.ToUpper()) && p.Index > node.Index ) ?? node;
        }
//Строим массив из TreeNode
private IEnumerable<TreeNode> GetNodeBranch(TreeNode node)
        {
            yield return node;
            foreach (TreeNode child in node.Nodes)
                foreach (var childChild in GetNodeBranch(child))
                    yield return childChild;
        }