There are tons of independent 409a providers, but the problem is that most charge absurd rates (for early stage startups at least), which is one of the ways vendors like Carta get startups hooked in and paying. You're basically getting "free" cap table software as a perk for paying for a reasonably priced 409a.

That said, as a data point, our startup recently did an independent 409a with EQVista, who had very reasonable pricing. They also have their own cap table management, but we don't use it.