Some VPNs kind of offer that. I know at least one that sells physical redeemable cards you can buy - maybe physically in some countries, but in mine it's only available on Amazon. Even that option should be safe for keeping your identifying data from the VPN provider, even in the situation where they betray their promises on not holding onto your data. This is because Amazon can't know which exact code was sent out to you, and the provider in turn doesn't have any additional info to associate with that code, besides knowing if it's valid or not. The biggest downside is that now Amazon knows you paid for this service, even if they don't know the specifics.
There's also an option to just mail them cash, but some countries may seize all mailed cash if discovered.