what should I ask in a shop to buy a magnet(not electromagnet )?
First choose a material.Black ceramic is weak but very cheap and not harmed by air, water, or handling.Neodymium comes in various strengths, from 'interesting' (N35) to 'dangerous in large quantities' (N52). Neodymium is more expensive, rusts to uselessness if its protective coating is damaged, and is killed by high temperatures.
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You're unlikely to find specifications for retail magnets, and even if you did I doubt that shop-assistants would understand them. I'd go with AndyGadget on this one. But If you're interested cheap 'n' easy linkL
It depends what sort of magnet you're after.
Most normal shops will only sell 'traditional' low strength magnets. Just ask for a magnet. If you are looking for strong magnets, shop on-line (especially eBay) for 'neodymium' or 'rare earth' magnets. They come in all shapes and sizes, and the pulling strength is usually quoted. They are also rated with an 'N' number. The higher it is, the stronger the magnet.
If you've got an old hard disk drive, pull it apart and you'll find a decently strong magnet in there.