That is a good question. You can see from the post I linked to that it is used by other software (in this case StoreFront), but I could not find anywhere on the web that talks about the DLL on its own. In that post, the author says, "AuthorizeNet's adcssl.dll component", but I have no knowledge of Authorize.Net having ever provided a COM DLL for accessing their service. Even if they did, though, it certainly is no available from them anymore.
If you are competent in classic ASP, you replace that code by calling another component or just call the service directly from VBScript on the server. Before spending any effort on coding for another component, though, you should first check with the hosting company to see if they are even willing to install it, since COM components need to be registered on the server where they are accessed. If they are okay with that, then there are a few components out there (including mine).
If, on the other hand, the hosting company will not install/register a COM component for you, then you can start with the ASP Classic Sample Code that Authorize.Net provides.
Lastly, if this coding is beyond your ASP/VBScript skill level, you could farm out that portion of the job. (Again, this is something I do and can discuss with you, but don't pick me just because I am contributing to this post. Find the person that is best suited all around for the job).
