Where can I get E180F tubes?

I have a Sabaj PHA3 tube headphone amplifier driving my Beyerdynamic DT770 Pro and Massdrop Hifiman HE4XX headphones. Using an iFi Zen Dac as the dac.

I would like to replace the two stock 6J9 tubes it comes with, with a pair of E180F tubes. Some say it brings a warmer sound signature to the amp.

Please advise