You can find it on amazon! It’s the absolute best. Here’s the link Seedlingers Plant Fertelixir 32oz Concentrate https://www.amazon.com/dp/B015EA6FXG/ref=cm_sw_r_cp_api_i_l2oyFb6T3M2AY
I've really never put too much thought into it but we do use this fertilizer that a fellow redditor recommended in a r/houseplants.
https://www.amazon.com/dp/B015EA6FXG/ref=cm_sw_r_cp_apa_fabt1_H52PFbB779JBH?_encoding=UTF8&psc=1