SHGetStockIconInfo is the correct way to do it, and doesn't require the addition of unnecessary file IO. It's not any more complicated than SHGetFileInfo.

Here is an example class structured in a similar way to Evk's class. Some important things to note:

1. When you get an icon handle from SHGetStockIconInfo (or even SHGetFileInfo, for that matter), the native icon must be cleaned up by calling DestroyIcon(), otherwise you'll create a resource leak.
2. When you create an icon using Icon.FromHandle(), the object stores the handle you gave it and will use it for later operations. This means if you immediately call DestroyIcon() and then try to do something with the icon you just created, it will cause exceptions. You can avoid this by using Clone() to get an Icon that doesn't rely on your original native handle.

public static class DefaultIcons
{
    private static Icon folderIcon;

    public static Icon FolderLarge => folderIcon ?? (folderIcon = GetStockIcon(SHSIID_FOLDER, SHGSI_LARGEICON));

    private static Icon GetStockIcon(uint type, uint size)
    {
        var info = new SHSTOCKICONINFO();
        info.cbSize = (uint)Marshal.SizeOf(info);

        SHGetStockIconInfo(type, SHGSI_ICON | size, ref info);

        var icon = (Icon)Icon.FromHandle(info.hIcon).Clone(); // Get a copy that doesn't use the original handle
        DestroyIcon(info.hIcon); // Clean up native icon to prevent resource leak

        return icon;
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    public struct SHSTOCKICONINFO
    {
        public uint cbSize;
        public IntPtr hIcon;
        public int iSysIconIndex;
        public int iIcon;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
        public string szPath;
    }

    [DllImport("shell32.dll")]
    public static extern int SHGetStockIconInfo(uint siid, uint uFlags, ref SHSTOCKICONINFO psii);

    [DllImport("user32.dll")]
    public static extern bool DestroyIcon(IntPtr handle);

    private const uint SHSIID_FOLDER = 0x3;
    private const uint SHGSI_ICON = 0x100;
    private const uint SHGSI_LARGEICON = 0x0;
    private const uint SHGSI_SMALLICON = 0x1;
}

I bet there are other ways, but I think easiest to implement is just use SHGetFileInfo win api function over temp folder you create. Example code:

public static class DefaultIcons
{
    private static readonly Lazy<Icon> _lazyFolderIcon = new Lazy<Icon>(FetchIcon, true);

    public static Icon FolderLarge
    {
        get { return _lazyFolderIcon.Value; }
    }

    private static Icon FetchIcon()
    {
        var tmpDir = Directory.CreateDirectory(Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString())).FullName;
        var icon = ExtractFromPath(tmpDir);
        Directory.Delete(tmpDir);
        return icon;
    }

    private static Icon ExtractFromPath(string path)
    {
        SHFILEINFO shinfo = new SHFILEINFO();
        SHGetFileInfo(
            path,
            0, ref shinfo, (uint)Marshal.SizeOf(shinfo),
            SHGFI_ICON | SHGFI_LARGEICON);
        return System.Drawing.Icon.FromHandle(shinfo.hIcon);
    }

    //Struct used by SHGetFileInfo function
    [StructLayout(LayoutKind.Sequential)]
    private struct SHFILEINFO
    {
        public IntPtr hIcon;
        public int iIcon;
        public uint dwAttributes;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
        public string szDisplayName;
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 80)]
        public string szTypeName;
    };

    [DllImport("shell32.dll")]
    private static extern IntPtr SHGetFileInfo(string pszPath, uint dwFileAttributes, ref SHFILEINFO psfi, uint cbSizeFileInfo, uint uFlags);

    private const uint SHGFI_ICON = 0x100;
    private const uint SHGFI_LARGEICON = 0x0;
    private const uint SHGFI_SMALLICON = 0x000000001;
}

Usage is just

var icon = DefaultIcons.FolderLarge

It's trivial to add property for small icon too.

public static class DefaultIcons { private static readonly Lazy _lazyFolderIcon = new Lazy(FetchIcon, true);

public static Icon FolderLarge
{
    get { return _lazyFolderIcon.Value; }
}

private static Icon FetchIcon()
{
    var tmpDir = Directory.CreateDirectory(Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString())).FullName;
    var icon = ExtractFromPath(tmpDir);
    Directory.Delete(tmpDir);
    return icon;
}

private static Icon ExtractFromPath(string path)
{
    SHFILEINFO shinfo = new SHFILEINFO();
    SHGetFileInfo(
        path,
        0, ref shinfo, (uint)Marshal.SizeOf(shinfo),
        SHGFI_ICON | SHGFI_LARGEICON);
    return System.Drawing.Icon.FromHandle(shinfo.hIcon);
}

//Struct used by SHGetFileInfo function
[StructLayout(LayoutKind.Sequential)]
private struct SHFILEINFO
{
    public IntPtr hIcon;
    public int iIcon;
    public uint dwAttributes;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
    public string szDisplayName;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 80)]
    public string szTypeName;
};

[DllImport("shell32.dll")]
private static extern IntPtr SHGetFileInfo(string pszPath, uint dwFileAttributes, ref SHFILEINFO psfi, uint cbSizeFileInfo, uint uFlags);

private const uint SHGFI_ICON = 0x100;
private const uint SHGFI_LARGEICON = 0x0;
private const uint SHGFI_SMALLICON = 0x000000001;

}