I use this utility, as you see it's modified for mscorlib or special assemblies:

Console.WriteLine(GetAssemblyPath("System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"));
// dumps C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System\v4.0_4.0.0.0__b77a5c561934e089\System.dll

...

public static string GetAssemblyPath(string name, bool matchClrVersion = true, bool throwOnError = false)
{
    if (name == null)
        throw new ArgumentNullException(nameof(name));

    string finalName = name;
    var aInfo = new ASSEMBLY_INFO();
    aInfo.cchBuf = 1024;
    aInfo.pszCurrentAssemblyPathBuf = new string('\0', aInfo.cchBuf);

    var hr = CreateAssemblyCache(out IAssemblyCache ac, 0);
    if (hr >= 0)
    {
        hr = ac.QueryAssemblyInfo(0, finalName, ref aInfo);
        if (hr < 0 && matchClrVersion)
        {
            var asmName = new AssemblyName(name);
            finalName = asmName.Name + ", Version=" + Environment.Version.Major + "." + Environment.Version.Minor;
            aInfo.pszCurrentAssemblyPathBuf = new string('\0', aInfo.cchBuf);
            hr = ac.QueryAssemblyInfo(0, finalName, ref aInfo);
        }
    }

    if (hr < 0)
    {
        if (throwOnError)
            Marshal.ThrowExceptionForHR(hr);

        return null;
    }

    return aInfo.pszCurrentAssemblyPathBuf;
}

[InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("e707dcde-d1cd-11d2-bab9-00c04f8eceae")]
private interface IAssemblyCache
{
    void UninstallAssembly(); // not fully defined

    [PreserveSig]
    int QueryAssemblyInfo(int flags, [MarshalAs(UnmanagedType.LPWStr)] string assemblyName, ref ASSEMBLY_INFO assemblyInfo);
}

[StructLayout(LayoutKind.Sequential)]
private struct ASSEMBLY_INFO
{
    public int cbAssemblyInfo;
    public int dwAssemblyFlags;
    public long uliAssemblySizeInKB;
    [MarshalAs(UnmanagedType.LPWStr)]
    public string pszCurrentAssemblyPathBuf;
    public int cchBuf;
}

[DllImport("fusion")]
private static extern int CreateAssemblyCache(out IAssemblyCache ppAsmCache, int reserved);