You'll get free lucky egg when you reach certain level. So far I already got 3 lucky eggs just from leveling up.

For mine, 1 at level 9. 1 at level 10 and the third one at level 15

i'm level 15 now btw

Level 2 - Poké Ball x10
Levels 3, 4 - Poké Ball x15
Level 5 - Poké Ball x20, Potion x10, Revive x10, Incense
Level 6 - Poké Ball x15, Potion x10, Revive x5, Egg Incubator
Level 7 - Poké Ball x15, Potion x10, Revive x5, Incense
Level 8 - Poké Ball x15, Potion x10, Revive x5, Razz Berry x10, Lure Module
Level 9 - Poké Ball x15, Potion x10, Revive x5, Razz Berry x3, Lucky Egg
Level 10 - Poké Ball x20, Super Potion x20, Revive x10, Razz Berry x 10, Incense, Lucky Egg, Egg Incubator, Lure Module
Level 11 - Poké Ball x15, Super Potion x10, Revive x3, Razz Berry x 3
Level 12 - Great Ball x20, Super Potion x10, Revive x3, Razz Berry x 3
Levels 13, 14 - Great Ball x10, Super Potion x10, Revive x3, Razz Berry x 3
Level 15 - Great Ball x15, Hyper Potion x20, Revive x10, Razz Berry x 10, Incense, Lucky Egg, Egg Incubator, Lure Module
Levels 16, 17, 18 - Great Ball x10, Hyper Potion x10, Revive x5, Razz Berry x 5
Level 19 - Great Ball x15, Hyper Potion x10, Revive x5, Razz Berry x 5
Level 20 - Ultra Ball x20, Hyper Potion x20, Revive x20, Razz Berry x 20, Incense x2, Lucky Egg x2, Egg Incubator x2, Lure Module x2
Levels 21, 22, 23 - Ultra Ball x10, Hyper Potion x10, Revive x10, Razz Berry x 10
Level 24 - Ultra Ball x15, Hyper Potion x10, Revive x10, Razz Berry x 10
Level 25 - Ultra Ball x25, Max Potion x20, Revive x15, Razz Berry x 15, Incense, Lucky Egg, Egg Incubator, Lure Module
Levels 26, 27, 28 - Ultra Ball x10, Max Potion x15, Revive x10, Razz Berry x 15
Level 29 - Ultra Ball x10, Max Potion x15, Revive x10, Razz Berry x 15
Level 30 - Ultra Ball x30, Max Potion x20, Max Revive x20, Razz Berry x 20, Incense x3, Lucky Eggs x3, Egg Incubators x3, Lure Modules x3