I'd expect even small scale legal firms to have their own domain name with email these days, but even then that doesn't guarantee anything. If you have a subscription to either MS or Google's business suite, you can get custom domain email for basically the cost of registering the domain name.

The only way I see your data not touching any US cloud provider is with a tech-savvy and privacy-conscious lawyer, and I suspect those are rare. Large firms may also still have their own infrastructure, but those are probably expensive as hell.