I think Natura 1600 is still made, twinlenslife.com has some great examples of Portra 400 at some remarkable speeds. Also, The Brothers Wright, who run twinlenslife.com, are starting to provide Kodak 500T with the remjet removed in 35mm, so you can process that plain old C41.

The word is Portra 400 is very similar to 500T, but I'd say 500T seems to do better in lower light.