You really DO NOT NEED a blind spot mirror. All you have to do is position your mirrors so that when you are sitting in the driver seat, you will be able to see the ‘blind spot’.
When you are sitting in the driver position, look in your left side mirror. Then simply tilt your head in order to make your left ear come as close to your left shoulder as possible. Then adjust the mirror so that you can barely see the left rear corner of the vehicle. Repeat this on the right side. After you have done that, you will be actually looking at the blind spot when your head is in the upright position.
You see, what happens is that for some unknown reason, when people position their mirrors they think that they need to see the rear corner, or even worse, the entire side of the vehicle they are driving. When in fact, the three mirrors will cover and overlap the viewing area.
So, after you have positioned the mirrors like I described, you will not need to move your head all the way around any longer. As you look in the mirror, you will not only see everything behind you and in your blind spot, your peripheral vision will also pick up vehicles that are beside you. Then you will be able to merge safely, and not have to take your eyes off of the road in front of you.