<telerik:RadButton runat="server" ID="RadButton1" Text="Get Selection" AutoPostBack="true" OnClick="RadButton1_Click" />

<telerik:RadTreeView ID="tvProcInst" runat="server" CheckBoxes="True" CausesValidation="False"
    DataTextField="LEVEL_NAME" DataValueField="LEVEL_CODE" RenderMode="Lightweight"
    CssClass="Left" MultipleSelect="True" OnNodeDataBound="tvProcInst_NodeDataBound">
</telerik:RadTreeView>

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
        If Not IsPostBack Then
            tvProcInst.DataSource = Enumerable.Range(1, 5).[Select](Function(x) New With {Key
                .LEVEL_CODE = x, Key
                .LEVEL_NAME = "Node #" & x
            })
            tvProcInst.DataBind()
        End If
    End Sub

    Public Function GetCB(ByVal id As String) As RadComboBox
        Dim cb = New RadComboBox()
        cb.ID = id & "_ComboBox"
        cb.Items.Add(New RadComboBoxItem() With {
            .Text = "Item 1",
            .Value = "1"
        })
        cb.Items.Add(New RadComboBoxItem() With {
            .Text = "Item 2",
            .Value = "2"
        })
        Return cb
    End Function

    Public Sub tvProcInst_NodeDataBound(ByVal sender As Object, ByVal e As RadTreeNodeEventArgs)
        Dim cn As RadTreeNode
        cn = New RadTreeNode()
        cn.Controls.Add(GetCB(e.Node.Value.ToString()))
        cn.Checkable = False
        e.Node.Nodes.Add(cn)
    End Sub

    Protected Sub RadButton1_Click(ByVal sender As Object, ByVal e As EventArgs)
        For Each node As RadTreeNode In tvProcInst.GetAllNodes().Where(Function(x) x.Checkable = False)
            Dim cb = TryCast(node.FindControl(node.ParentNode.Value & "_ComboBox"), RadComboBox)
        Next
    End Sub