For perspective, the "scale" of biomes in a default world is 16x16 chunks, or 256 chunks per biome (random noise used during the zoom stages makes them irregular but it can be expected that this is their average size); a probability of 0.1% means that on average you need to explore about 256,000 chunks to find one, or an area about 8096x8096 blocks (for comparison, this is about twice the area of my largest world). However, due to the way biomes are placed they are much easier to find if you are within a "hot" climate zone.
Say something silly, Laugh 'til it hurts, Take a risk, Sing out loud, Rock the boat, Shake things up, Flirt with disaster, Buy something frivolous, Color outside the lines, Cause a scene, Order dessert, Make waves, Get carried away, Have a great day!
I've seen a few before, and they're actually fairly easy to find if you just find a large savanna and follow it. Also Conor, you can find a shattered savanna in any world by typing /locatebiome shattered_savanna