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.Then choose a size and shape. Unless you are buying the magnet for sheer coolness value, do not buy anything larger than you need and definitely do not buy anything larger than you can control.Cylinder magnets are usually a better deal for long-range pull. They don't have corners that can break off. But rectangular ones can have very small but concentrated spots of magnetic force at the edges and corners, and of course have more surface area at the ends.Finally, for a given size, you have a trade-off between short-range pull (flat) and long-range pull (long).A magnet that is shorter from pole to pole will stick more strongly on thin metal, while a magnet that is longer from pole to pole will have a more far-reaching field.Those paper-thin refrigerator magnets have north-south stripes, so they can be as strong as possible through a few sheets of paper, but because of this are totally useless at holding thick objects.Suppliers:Applied Magnetics has a price guarantee, which is good for large magnets. K&J Magnetics has a wide selection, which is good for small magnets.