Try eBay, though you may want to search for "whelen."Is there an LED Replacement for the Whalen A610 strobe tube? The A610 is almost $200.TIA
you need to check one more piece of the puzzle, the STC that applies to the instillation of the strobes. The STC could have been approved with all three installed and no option to install just two even if the light pattern covers the regulation.Try eBay, though you may want to search for "whelen."
If thinking of replacing the tail strobe, check out:
https://www.aveoengineering.com/
On the other hand if you have wingtip strobes you may not legally need the tailtip strobe:
https://www.dropbox.com/s/0ji5jdpu6zksndh/Anti-Collision Systems.Regulations.pdf?dl=1