At Lowes the other day, someone recommended the TrimmerPlus Add-in with Aero-flex head (AF720). This would fit my Troy bilt 4 cycle. It takes 4 plastic blades that seem easy to swap out, but the cost may be high depending on how quickly they break. A replacement pack of 16 is $12. May be a lot cheaper to use one where I can cut my own string; but on the other hand, it's probably going to be a royal pain to replace the head to get things going.
Too bad there's no string trimmer attachment that simply comes with a standard, non-bump head that takes individual lengths of string.