where do I buy replacement seats for iron garden chairs?

I take it you are refering to wood seating.
Can't tell you that you don't have a makers name and there are loads of different design seats.

You could take a digital picture of one of the chairs and close up of the seat area show that to someone in garden furniture centre you might be lucky.
Failing that take measurements of the seat area to a lumber yard [timber in England] and get them to make them, of course they will be flat with no indentation for the bum, just put throw cusions on them to soften the blow when you sit down.

Bob
April 2010