When you say "host header," I assume you are referring to the web application to which your site collections belong. With that in mind, you can use the SPWebApplication's AlternateUrls collection to get the host header(s) associated with that web application.
Since it is possible to have multiple URLs associated with a single web application, you would need to iterate through this collection to get/find the host header you want. For instance:
SPAlternateUrlCollection allUrls = SPContext.Current.Site.WebApplication.AlternateUrls;
foreach (SPAlternateUrl url in allUrls)
// Do something with each url.Uri.ToString()
Note that you will need to add a reference to Microsoft.SharePoint.Administration to make use of this code.