I asked a professor I had 6 years ago for a letter and got a great one from him. I've also had contact with a bio prof from 1998 who was very helpful in sharing his notes for a class I taught. It's not easy, and you might have to be a little persistant. If you can go in person, that's a bonus! If you have the money to take more classes, go for it, though, because it's only going to help the more upper level sciences you have. I have taken upper level classes for the double purpose of 1) having the A in the upper level class and 2) getting the letter from the prof. You know, I would ask the old profs and take the class if at all possible. Then, if you get the letters from the old prof, you have letters sooner, and if not, you've got back up! Good luck!