Anyone in the Pattaya/Najomtien area know where I can buy NPK fertilizer at a decent price I usually get 20 KG, but the delivery cost on Lazada is too much.

I don't know Pattaya but HomePro, Global House, ThaiWatsadu and other home improvement superstores usually have it in the garden section. The plant shops outside in parking lot areas of some HomePros often have it too, as well as ag specialty shops in farming areas.  

 

My two satang as an organic program advocate, is to avoid repeated use of high NPK, high salt index chemical fertilizers. They may jack green growth that looks good, but do not supply complete plant nutrition and do not contribute to long term soil and plant health. High salt index chemical fertilizers like 15-15-15 dessicate roots, kill the all-important beneficial soil organisms, and create internal plant biochemistry that attracts pests and disease. If you go that route and find your NPK fertilizer, pick up some insecticide and fungicide too, you will need it. 

 

Or go instead with complete organic fertilizer like that from Organic Totto, Best Garden State, or Maruchu Bussan.