How do I switch to Latanoprost from Latisse?

I would like to switch using brand name latisse to use of Latanoprost because I hear it is cheaper. I live in Mexico, and prescription is not required here. But I do not know how to ask the pharmacy for Latanoprost. How do I ask for this? Is there a "percentage", do i need to specify I want a "solution"? How is the prescription written? Thanks so much!