Charlton Athletic have announced that they will be backed by 38,227 fans at Wembley on Sunday.
The Addicks cleared their initial allocation of 36,918 earlier in the week as extra tickets were made available.
They have now taken ticets off sale ahead of the play-off final.
Tickets for the Sunderland end are still on general sale, though the club have advised that they cannot be purchased in person from the club ticket office past 5pm today (Friday).
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Tickets will remain on sale via Ticketmaster's online box office until 5pm Saturday.
Supporters purchasing in this time can print their tickets off at home.
Tickets are no longer available via telephone.
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Sunday's game could yet break the record for a League One play-off final.
The record currently stands at 76,935, when Manchester City famously edged out Gillingham on penalties in 1999.
Sunderland revealed earlier this week that they had sold 33,000 tickets, shortly before the start of the general sale process.
The Black Cats were initially handed an allocation of 37,700, though that was subsequently increaded.
Executive Director Charlie Methven urged any 'wavering' supporters who could to make the trip.
"Whether you are a player, a supporter or a member of staff, Sunday’s game marks the culmination of our efforts throughout the last 12 months," he said.
“Sunderland supporters have consistently been at the forefront of the club’s efforts, following the players every step of the way on a journey that, poetically we hope, will start and end with wins against Charlton Athletic.
“The Checkatrade Trophy was a party, but this weekend is a different matter, I think. It is about us all coming together one last time and doing everything within our power to push our club over the line.
“Given the short notice, of course there will be people with valid reasons for not being able to attend," he added.
"I would, though, urge any waverer who can come to Wembley but hasn’t yet bought a ticket to do so, and be part of making the crucial difference.
“All of us football fans know that there are only so many matches that have the potential to become ‘I was there’ memories, and Play-Off Finals certainly fit into that category.”