A list of sources for Lizalfos Tails, which are used in upgrading the Soldier's set of armor.
Best Source of Lizalfos Tails
You can purchase Lizalfos Tails from Maike in the Kara Kara Bazaar. She sells 3 at a time. If you go some place far away from Kara Kara Bazaar and sleep or wait by a fire until morning, her shop will restock.
Other Sources of Lizalfos Tails
The only other way to get Lizalfos Tails is to defeat regular (green) Lizalfos. There is only a 25% chance of getting a Lizalfos Tail from defeating a Lizalfos. They are common in the Lanayru Area, which includes the area from Lanayru Wetlands to Zora's Domain.