Autumn colours and themes are lovely - I've done a couple over the years.
However, you'll probably find them hard to find on many cake websites, they're not something I'd put on mine, for example, as they don't really have the wide appeal of the more standard pink/white, gold/ivory etc. I, like many cake makers, make too many cakes to be able to put them all on my webite so have to think about what will have wide appeal that I'm likely to get business from.
That said, I just searched for 'autumn wedding cakes' in google images and quite a few came up in shades of ivory with browns, russets, golds etc. Most of them seem to be on websites in the USA but if you just need them for inspiration to show your MIL2B then it's probably worth searching that way.