The icons I want don't seem to be available from the "Add or Remove Buttons" routine in Excel.
If the commands you want are available through the menus, then they're probably available under the "All commands" list when you go to customize the toolbar. I've only got Office 2007 available here, so I can't confirm the exact wording.
You'll have to make do with one of the (naff) stock icons: I don't think Excel 2000 lets you design your own.
If that doesn't work, record a Visual Basic for Applications (VBA) macro that does what you want, and then assign that to a button.