For anyone interested:
I ordered some Mikuni jets recently and decided to ask for the Formula 3 carb kit needles (5EJ48) as well, just on the off-chance (I had always thought they were long since discontinued). Imagine my surprise when I was told they were in stock!
I decided to wait until they arrived, just in case there had been some misunderstanding. However, I have now received them and they appear correct.
I bought them from Mikuni Topham in Germany. The price was 11 Euro each.
All of the other Formula 3 carb kit parts are still fairly easy to get (the needle jets are 166-series I think, although 159-series should also work), so anyone wanting to convert their standard carbs should have no trouble.
Regards,
James