Resistance bands have their place, but they won't replace proper weights. I think p90x uses dumbbells and bodyweight exercises, so your best bet would probably be to get an adjustable dumbbell set, and adjustable bench and a pull up and dip bar. I've never done p90x before, and no one ever seems to write what specifically they do in each training session whenever I find a training journal that includes p90x, so I can't make any objective claims about the program, but I hear that it's not completely awful, so it's at least better than nothing.
Other options on good equipment are kettlebells and an Olympic barbell + weight set. A barbell + weights + good stands/uprights/a squat rack/power rack will set you back a fair bit financially, and it's probably the most expensive option out of the 3 top options (barbells, dumbbells and kettlebells), but it's also one of the most rewarding, especially for lower body exercises (you'll never squat or deadlift with a dumbbell or kettlebell the sorts of weights you can handle with a barbell).
