I think Hawthorn or hibiscus. You can get them certified as USP or cGMP, and/or independently tested by Consumer Labs.
I'm not 100% sure on the mechanism of action on Hawthorn. I've read that it's an ACE inhibitor, but I don't have a good source. But I can see at the very least that it's an elastase inhibitor, which is supposed to be protective against ARDS.
edit: I think proanthocyanidin is the component that is supposed to be an ACE inhibitor.
I'm just taking some expired Hawthorn pills that I happened to have lying around the house. In my case I don't have high blood pressure to begin with, so I'm just taking about 1/3rd of the recommended dose and then periodically checking to ensure that my blood pressure isn't getting dangerously low.
People make their own hawthorn jam and jelly though, so it isn't excessively dangerous, but on principle it's probably better to take a conservative approach, or at least one that's commensurate with the initial risk.
Because I'm in NYC we actually have them growing everywhere because they are one of a handful of approved small-sized street trees, and they're in all the parks, but regardless it's easier just to buy as a supplement.
Edit: Luckily I own several acres of Hawthorn bushes.