Add text to first available cell in range with non-contiguous cells

I have a textbox in a userform, for entering comments. The data should be entered into one of the following cells C44, C47, C50, C53 and C56.

How do I write the data into the first available cell starting at C44? If the cell has data, move to the next cell and so on.

I managed it only for C44.

Private Sub CommandButton1_Click()
Range("c44").Value = Date & ", " & "št. sarže: " & ActiveCell.Value & vbNewLine & TextBox1.Value 
Unload UserForm10
End Sub

Here's an example to find the first available cell and write data into it. If all cells are already filled, displays a msgbox.

Private Sub CommandButton1_Click()
    Dim rng As Range
    Set rng = FindFirstAvailableCell

    If Not rng Is Nothing Then
        rng.Value = Date & ", " & "št. sarže: " & ActiveCell.Value & vbNewLine & TextBox1.Value
        Unload UserForm10
    Else
        MsgBox "All target cells are filled.", vbExclamation
    End If
End Sub

Function FindFirstAvailableCell() As Range
    Dim targetRange As Range
    Dim cell As Range

    Set targetRange = ThisWorkbook.Sheets("Sheet1").Range("C44,C47,C50,C53,C56")

    For Each cell In targetRange
        If cell.Value = "" Then
            Set FindFirstAvailableCell = cell
            Exit Function
        End If
    Next cell
    Set FindFirstAvailableCell = Nothing
End Function