googles

This says that they restarted production, if you're in the US.

https://mountaindew.fandom.com/wiki/LiveWire

Current Status: United States, New Zealand, and Australia: Permanent Flavor

Year(s) Available: United States: 2003, 2004, 2005 - Present

LiveWire is a Mountain Dew flavor released in stores in the summertime in 2003, 2004, and then 2005 for a limited time in the United States. During its release, it proved to be massively successful. This introduction alone increased overall Mountain Dew sales by 10% in its first year, eventually making LiveWire a permanent flavor after its 2005 limited-time run.

Walmart says that they stock and can ship it:

https://www.walmart.com/search?q=Livewire

Might just be that it's not showing up in local stores near you.