Emery Davis wrote:

Around here the farmers use chestnut or acacia for fence posts, because
it doesn't rot
If I could find machined logs, I'd build it with that rather than the
oak. Probably less dear too.

The semi-local supplier does offer English Larch or Douglas Fir
sleepers, which they say don't need treating due to high resin content
of the wood, they would be cheaper if they sold 50mm in the Larch like
they do for oak, but they only do the Larch in 100mm thickness, so it
works out slightly more expensive.