You might like the Røde NT-4 stereo mic. It has true condenser capsules and can run from an internal battery. I know from personal experience that it is compatible with the M10 (with proper 5-pin xlr to stereo 3.5mm male cable). I see them offered used for under $300US, so it would fit your budget. It is a bit heavy, though, and as is usual with any directional mic, will need a suspension if you plan to hand-hold, and a good windscreen if you plan to record outdoors. Best of luck!