A number of CAs are working on ACME server implementations, based on posts on the working group mailing list. There are a couple of reasons why none are availa

A number of CAs are working on ACME server implementations, based on posts on the working group mailing list. There are a couple of reasons why none are available today. First, ACME isn't quite standardized yet. Let's Encrypt currently implements an older draft and will start offering an ACME server running what will become the RFC version of ACME early next year. It makes sense for other CAs to wait for the standardization process to finish (hopefully while working on their own implementation and giving feedback!) rather than implement the draft currently supported by Let's Encrypt and all clients, and then migrating to an incompatible new version soon after. Going live with a server supporting the latest draft wouldn't make much sense either since clients wouldn't work with it. Another reason is that the later drafts saw a couple of additions and changes that are relevant for commercial CAs, for example things like the ability to bind ACME accounts to existing CA accounts and around the way out-of-band processes are handled (payments, validation steps that cannot or must not be automated). This was done based on the feedback from a small number of commercial CAs that were active in the working group and should hopefully make ACME a viable option for many others.