You can get hardwood plaques at craft stores like Micheal's or Hobby Lobby. You have to stain and seal them yourself, but w/ a little care they can look pretty good. Use polyurethane to seal them, it's water-proof.
In the alternative, you can just paint them w/ a good sandable primer, sand them smooth and paint them the color of your choice.
If you're in the mood for something simple and easy to do, I've used PVC plumbing connectors as bases before:
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Plug one end w/ styrene sheet, and them fill it w/ your choice of material to give it heft (I prefer casting resin or old fashioned sand), then seal the other end. Good for column style bases.
If you want a dedicated model base, it's relatively easy to make you own from styrene sheet. I posted some pictures of a simple base of this type in this thread:
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Good luck w/ whatever you choose.