I think that any blood will do, but let's wait for someone else's opinion.
By the way, why is it easier for you to draw blood from your legs than from your left index finger? What method do you use? Let me give you a few tips you can try.
First, a sterile one-usage needle is best. Before trying to draw your blood, make sure you are calm and that your hands are not cold. When actually trying to draw the blood, put your index finger between your thumb and middle finger and squeeze the tip of your finger between them. Now, all you need to do is prick your finger a bit. It's completely harmless, I have done it many times and you should feel a small pinch.
There are different types of needles. For example, you wouldn't want to work with a needle used for drawing blood from your veins. It's supposed to be hollow so it's too thick. If you have access to small needles used for insuline injection then you're good to go.