Hi Torbion
I've made them myself (cut from oak planks with a routered edge), bought them from eBay unvarnished and still needing to be finished (search for model display bases) or from Oakwood Studios who makes bespoke bases at a very reasonable price. The owner, Richard, is always very helpful will make exactly what you need if one of his standard bases isn't quite the correct size and they arrive very quickly beautifully finished. They are also work out very good value compared to the cheaper eBay items
Cheers
Paul