Infinity and Nissan share many identical part numbers. For your '01 QX4 and my '02 Pathfinder, the OEM part number for the rear springs is 55020-1W201. I purchased ACDelco springs for my ride and they worked out great.
https://www.amazon.com/ACDelco-45H2158- ... 007QBY0GQ/
RockAuto has them as well. If your springs are shot your upper and lower insulators are likely worn out as well. The lower insulators are just thin wall sleeves that fit onto the lower spring coil and they get abused the most over time. OEM number 54034-2J010 is the only option for those lower insulators I could find. OEM number 55034-0W005 for the uppers I found available as aftermarket but I went with OEM instead because of fitment issues with a lower cost alternative I first purchased. As for the springs, there are lower cost alternatives to ACDelco but I have seen their brand generally higher in quality and reliability than others so for me it was worth paying a bit more to be confident I would have no more issues with this for years to come.