There is a store locally that sells surveyors equipment, it's where I go with questions like this, find one near you, it may be helpful for future reference.
They say "Old school PK" nails are on the way out" I find them sufficient. They can still be purchased most places if they have old stock. The new standard is the MAG nail, a little more rugged and has a larger magnetic signature, it's likely what you'll find mostly in-stock now. Eiter work fine, PK nail, if you can find them are a little cheaper.