I use whatever's hanging on the refrigerator at the time I need one - Pizza Hut card, baseball schedule, etc. It doesn't have to be very big or anything fancy. Remember the idea is to have the magnet not stick if there's bondo so the less powerful the better. Try it out on your own car to get a feel for it. ::
the best ones are the soft flat ones, the cheap ones you get from companies advertising. They are not very strong so they show even light bondo.
If you really want to get fancy, look for a old style stud sensor. this is a magnet on a pivoting frame used to find the nails in studs. they are still sold but harder to find these days. This is good for finding smaller amounts of bondo.
I actually take the opposite approach of what the others have said.
I have a telescoping 1/8 high powered pick up magnet I use to check for bondo. I take an old t-shirt with me and wrap the tip of it to avoid scratching any paint and slide it all along where I want to check. When I feel a change in attraction I inspect that area closer
Make sure you have a piece of felt or something soft that won't scratch the paint in between the magnet and the car. I've seen alot of folks glue a thin layer of felt to the bottom of a magnet just for this reason. I would never agree to let someone check my car with a fridge or bare magnet even if it meant loosing the sale!