Lead is not PC
Use gold and platinum. By the time they get it launched the original cost of the material wouldn't matter.
Not my idea...read it in a book...but they could use huge tanks of water. Maybe get the water from the moon now that they've found some. Then once your at Mars and have buried your habitat beneath a suitable amount of dirt to block most of the radiation....BOOM....you have enough water to for the mission and beyond. The necessary thickness of water is more than lead....but there will be a use at the end....plus can even the environuts whine about using water?