Rear suspension problem/question

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I have a 98-Infinity QX4 and the rear suspension bottoms out very easy, I replaced the rear shocks but it continues so I now think it's the rear coil springs.I have check a couple of auto parts stores and they don't carry the springs so I went to the dealer and I can get them from them but they were expensive, but I was really wondering is

1) Does someone else (not infinity) make a better spring ? Do you have their webb address?2) Is this something I can do on my back or should I take it to a shop to replace?

Thanks.