Are you asking what the main food is of Spanish-speaking people? Or are you asking how to say the words "main food" in Spanish? Please clarify your question. But the main food of Spanish-speaking people is different from one Spanish-speaking country to another.
There is a syntax issue here. You really can't say "I can buy you" in Spanish unless you are talking about actually purchasing someone. What you mean to say is "I can buy food for you". "Puedo comprar comida para ti" is how you would say that.