This is a paid asset, but now you can download Awesome Game UI Pack, Button Pack and Icon pack 16 for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
Awesome Game UI Pack Button Pack and Icon pack 16 v1.0
Full UI kit for the game. UI Panel, Button, Icon, and much more. This asset pack contains mobile-friendly Buttons, Icons, panels, and other UI objects.
Awesome Game UI and Icon pack 16
Features:
400+ different buttons
5 different colors in buttons
83 different white icons
12 Popup background
82 probs/Items
28 Ribbons
Much more to come
Button sizes are 512*512
Also, look at the pack
Awesome Game UI pack 8
Awesome Game UI pack 9
Awesome Game UI pack 10
Awesome Game UI pack 11
Awesome Game UI pack 12
Combo of Awesome Game UI pack 8, 9, 10 and 11
Combo of Awesome Game UI pack 8, 9, and 10
Combo of Awesome Game UI pack 9, 10 and 11
Combo of Awesome Game UI pack 8, 10 and 11
Combo of Awesome Game UI pack 8, 9, and 11
Button sizes are 512*512