This gist allows you to get a master page of a given type from a page. It loops through all parent master pages until the given type has been found.

PageExtensions.cs

using System.Web.UI;

namespace Extensions
{
    /// <summary>
    /// Provides <see cref="Page"/> extensions.
    /// </summary>
    public static class PageExtensions
    {
        #region Methods

        /// <summary>
        /// Gets a master page.
        /// </summary>
        /// <typeparam name="T">The type of the master page.</typeparam>
        /// <param name="page">The page.</param>
        /// <returns>A master page.</returns>
        public static T GetMasterPage<T>(this Page page) where T : MasterPage
        {
            T masterPage = null;
            MasterPage currentMasterPage = page.Master;
            while (masterPage == null && currentMasterPage != null)
            {
                if (currentMasterPage is T)
                {
                    masterPage = (T)currentMasterPage;
                }
                currentMasterPage = currentMasterPage.Master;
            }
            return masterPage;
        }

        #endregion
    }
}