A lot of schools are pretty strict requiring letters from science professors.
Are you still in school and if so, how long do you have until you graduate?
Are you taking science classes right now? If so, you need to start taking advantage of those office hours, or approaching them immediately after class to introduce yourself, and ask questions.
Don't let inertia bite you in the butt and then come here complaining about needing science LORs when you have in it within your power to take the initiative by approaching those science professors and start building a instructor/student relationship.