A few years ago I bought a heavy ceramic planter saucer from my local garden center that I frequent, and it made a wonderful bird bath. It was actually sloped like a regular bird bath, a bit deeper in the center, and was the perfect depth and price. It was 18" and only cost $15 (I think it was a 20% off sale, but still cheap compared to bird baths). I'm not sure how you'd attach it to a fence post though, that wide the ceramic is heavier, which is what I liked about it.
I just used it on the ground with planters around it. There's also shorter black wrought iron stands you could place it on top of, those wouldn't need to be an exact fit, and again those were much cheaper than a bird bath pedestal.
