I’ve found a few so far, but most of the free assets aren’t great. Assets on the unity asset store can be good though but there’s just not enough to necessarily build your demo. Also need them to be a similar art style too
There’s quite a selection of free assets in FAB, covering different art styles. You just need to setup your filters, in order to locate what you are looking for.
As a sample, stylized art is quite popular these days, by setting up filters like this, you can stick to said art style only:
And here are a few recomendations from that search: