Our own stores sell rivets with a 2mm head:-
Rivets with a 2.5mm head are currently out of stock, but I’m sure our Trade Officer will advise when/if a fresh supply is anticipated.
But why rivets? Using the functional plastic chairs (also available from our store) provides a much better appearance with less hassle. If you’re concerned about the alignment of rails at the crossings, solder small straps of 3mm x 0.5mm brass to the underside of the crossing and glue them to the timbers.