I am working a tutorial for it now but in a nut shell you want to look for "rare earth magnets". The most common type for vehicles are the 1/8" while infantry models use 1/16".
Once you are looking at the mangets getting ready to buy them you will see a number that is preceded by an N, like N35, N40, N52, etc. Those are the different stengths, the highest strength you can get is an N52 magnet. For general use I find that a N40 is normally good.