Email every professor you have ever had in the science/math discipline individually if you got a A/B in the class. Explain your situation (I am applying for X for X admission, and they require X), and ask if they would be willing. If you are close to campus and can get their schedules, going in person may not hurt either.
I was in the same boat - and I'm still in school. I never get to know my profs really well because quite frankly I don't usually need office hours. I just started emailing everyone I could remember ever having during college, and I got someone who was willing, went into his office hours, gave him my resume/statement of purpose, and he wrote a really good one for me. Most professors who you have gotten an A/B for will be happy to unless you did something to really tick them off.