Where can I find System.Web.HttpContext.Current.Server dll?

System.Web.HttpContext.Current.Server isn't a namespace. Let's break it down:

• System.Web is a namespace
• HttpContext is a type (in the System.Web assembly)
• Current is a static property within HttpContext, returning an HttpContext reference
• Server is an instance property within HttpContext, of type HttpServerUtility

So the assembly you need is System.Web; the namespace you need in a using directive (if you want one) is System.Web.

So:

using System.Web;
...
var path = HttpContext.Server.Current.MapPath(...);

Note that a more modern equivalent is HostingEnvironment.MapPath, which doesn't rely on a current HTTP context, as far as I'm aware. (HostingEnvironment is in the System.Web.Hosting namespace, but still in the System.Web assembly.)