Since I experienced it myself how hard it is to find these expanded clay pellets also called Hydrotons or Hydrocorns, much more finding an offer with reasonable price, I will share where I bought mine.
Establishment: Aya's Garden
Price: Php 100.00 per kilo (retail)
Class: Large Pellets
Address: Manila Seedling Bank, Quezon Ave. corner EDSA Quezon City
I bought a kilo because it is needed for a hydroponics project called Window Farm (which I haven't started yet and not sure when to commence) that I watched in YouTube.
Searching the internet and our local trusted classified ads website, www.sulit.com.ph, sellers there offer it for a ridiculous price of Php 180.00 per kilo. Woah, that's expensive.