Is your Nintendo 64 controller joystick loose and worn out? If so, you will be pleased to know that it is very easy to replace a loose joystick with either a replacement one, or a good one from a donor controller. In total, this swap will take about 15 minutes if you're switching joysticks between two controllers, or half that if you have a replacement.
I have always used joysticks from used Grey or Atomic Purple controllers, simply because they are plentiful and cheap as most people tend to use the colored controllers more often than their "boring" brothers. I have never used a replacement joystick, so I cannot comment on their quality.
After the swap, you will have an N64 controller with a nice tight joystick, and if you used a donor, one with a loose stick. I will usually sell the loose controllers for what I paid for them, around $5, so that no one feels ripped off. The buyer gets a cheap official controller, and the seller gets his money back.