Are you talking about diesels? Drop grab irons are the most common type. If you're looking at installing some on your models, there are a couple manufacturers (like A Line) who make very nice ones. As for most others, I would just make my own with 0.015" brass wire.
EDIT: Just saw your last post, and noticed you're talking about N Scale. BLMA makes some nice looking pre-bent grab irons (straight and drop) out of super fine 0.007" wire.
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