There's no way for you to book a room and tickets separately if you want a dining plan. Dining plans are sold as a package add-on and are not available without a package booking (except for DVC members booking through DVC member services).
There are a couple of exceptions to the ticket purchase requirement for packages - annual passholders and those eligible for the military room discount can book ticketless packages. But if you're just looking to get a room only reservation and then pick up your tickets from a discount broker, you can't do that and get a dining plan. You would still have to book a package with at least a one-day ticket purchase, and the way Disney ticketing is set up, it's not worth it to get a one-day ticket if you really want a multi-day ticket (unless the dining plan is important enough to you that you are willing to get the one-day ticket and save it for future use).
Even on a package reservation, the dining plan must be added to the reservation prior to check-in. 48 hours is for DVC reservations. It's 72 hours on packages.