Firm paper and Bristol board refer to the material of which these are made. The cut pieces themselves are correspondence cards (approx. 15x10 cm), and when used for study notes they become flashcards (to show to others) or revision notes (for your own use).
You're right, but the card is the same size and thickness.
Revision notes, which is probably the term you're looking for, can of course be written on any size or thickness of paper, a laptop, an electronic notepad, the kitchen wall, the cuff of your shirt, the back of your hand...
I know this is not likely used in India, gengowr, but for other readers in the future, in AE those are known asindex cards, or sometimes called by their usual size in inches: 3 x 5 cards (three-by-five).
The term card reflects the heavier paper used.
Flash cards would only be used for a specific educational purpose: when the index cards carry a question or prompt for a student, with the answer written on the other side. That purpose is more specific than using the cards to make study notes.
The palm of your hand surely - where the exam invigilator can't see them.
gengowr wrote "mainly at school". Ouside school they are used for other things and given a name according to use.
(When doing academic research - before there was the required computer software - they were called "reference cards" because they were used for noting details of references to academic papers. The list of possible names in English must be very long).
Because of that, to find them in a stationery I suppose one asks for Bristol board cards with the dimensions required - but I'm not sure.
PS I think in a stationery I would be tempted to ask for Wildan's "index cards".