If you told them you needed a rack & pinion boot, then, "yes," the boot, or "bellows," would be available. But did they actually see the part you are talking about? If not, then they probably didn't realize what you were talking about. According to the parts diagram, the boot isn't available separately (NOTE: copy and paste the entire link in order to get it to work, both the highlighted and non-highlighted part...rather than click on the hyperlinked part of it):
https://www.nissanpartsdeal.com/Page_Pr ... &PNC=48830
The joint assembly has a bolt and not at the bottom and there is a pinch bolt and nut located at the top. Once removed, one should be able to slide the lower shaft up enough to disengage the lower joint assembly and then the joint assembly can be removed. It is important to remember that the steering wheel should be held in place in some method to prevent it from turning freely, which could result in damage to the clockspring wires if it is installed out of sync. I usually center the steering, first, then tie a string or wire between the brake pedal and the bottom of the steering wheel to give it some play but not enough to turn a full revolution.
There is no grease inside the boot; it is just to protect the joint inside of it. Since you are experiencing hard steering, the joint is likely binding and no longer good, as I mentioned before, so, even if the boot was available, it's still not going to fix the problem of the bad joint.