What do you call it and where to buy some
I don't think there is an English word for these. It seems as though we just call them by their Indian name, of which there are several. Pappadam and Appalam seem to be the most common names, although in different regions of India the name can vary slightly. In the picture you posted these are referred to as Round Chips, at least according to one merchant.

A recipe can be found at Awesome Cuisine:

Ingredients:


Black Urad Dal (black lentil flour) – 1 cup 
Salt as per taste
Cumin Seeds – 1/2 tblsp
Cooking Soda (baking soda) – a pinch 
Oil – 1/2 cup 
Asafoetida Powder – a pinch


Method:


Clean and spread out the urad dal under the sun for a few hours.
Then grind to a smooth powder.
Add asafoetida powder, cooking soda and salt.
Add enough water and knead well to a thick dough.
Add oil and cumin seeds.
Knead again to a chapati dough consistency.
When the dough starts to turn pale yellow, pinch small balls of the dough and roll them out into thin circles.
Keep them under the sun (or under the fan) for 30 to 45 minutes or until the moisture has evaporated.
Store them in a clean, dry container.
Fry them in hot oil and serve with rice or as desired.