It is better to buy car parts online if you don't need the car parts right away.
If you need the car parts right away then it's better to buy them at an auto parts store but if you can wait for the car part then it's best to buy the car parts online as they can be cheaper than buying car parts in store.
The best place to buy auto parts online is RockAuto, eBay, Partsgeek.com and Amazon.com
These are the places where I buy my auto parts from.
Most auto parts I buy from eBay although sometimes I can only find the auto parts I need at RockAuto, Amazon or Partsgeek.
You can get most auto parts online cheaper than you can in an auto parts store.