30,000 expected at Wembley
A Millers spokesman said: "The response has been fantastic and we hope there will be another rush of sales in the run-up to the deadline.
"It would be fantastic to see 20,000 Millers down at Wembley."
Dagenham, whose average league gate is much less than Rotherham's, have put out appeals for support right across London. Their numbers will be swelled by a large contingent from neighbouring West Ham United.
The deadline for bookings is 10pm tonight.
All bookings are being handled by Ticketmaster online and by phone.
Prices for adults start at £28 and climb to £38, £48 and £60, with half-price concessions available.
Both teams, meanwhile, are expected to be unchanged from their play-off semi-final victories.
Nick Fenton (calf) has missed two days' training but trained yesterday and is expected to be OK.