If you don't mind going with two boosters instead of one, you could probably fudge with the boosters from a Revell Snark kit; it has two large ones that might scaleorama down nicely for 1/48 (odd note: when I was doing the booster rigging for Army-TSSAM, which also had two boosters, I was able to reference the rigging instructions for the Snark from someone there at Northrop who had worked on it).