Tickets have been on sale for season ticket holders only until recently and now they have gone on sale to associate members and registered supporters of the Midlands based club, meaning only a limited number are likely to remain for general sale.
"Ticket sales for the Heineken Cup quarter final have been excellent thus far and we continue to thank Season Ticket Holders for their ongoing support," said Saints chief executive Allan Robson.
"We have had plenty of enquiries from Associate Members and Registered Supporters over the past two weeks and now they have their own period of priority booking.
"The only way that we can guarantee that the Saints tickets go to Saints supporters is if Associate Members and Registered Supporters book tickets, and for those Season Ticket Holders who have not yet booked to do so, and get tickets for their friends and family as well.
"We are preparing for tickets to go on general sale, but we won't know for certain until the end of the week, so the advice is for supporters who want to order tickets to keep checking the club's website or sign up to our eZine to get the information as it is released."
Northampton Saints were the only side to finish the Heineken Cup pool stage unbeaten, winning all six matches in Pool One.
Ulster qualified for the knock-out stages for the first time since winning the competition back in 1999 as one of two best runners-up, finishing level on points with Biarritz Olympique in Pool Four.