If the buzzing is happening on only those two frets, then it means that the neck is warped. If you go to any music shop they should be able to adjust the truss rod for you and straighten the neck back out, which shouldn't cost more than $20.
Also, your strings should be changed every month or so (maybe longer if you're not playing very often), so you should be able to change them yourself, it's not very hard to do.