Gotta be specific bud. Year, trim, etc.
You can always try Amazon.com automotive... I think the *real* question here is whether you're looking for an aftermarket shift knob that is 'universal' or one that is threaded to match your particular car's shift lever?
Ive only shopped around once for a shifter for my 93 sentra and the best place was a local Bazaar which I paid $13 for a blue see-through knob with white waves of ribbon through it. Looks sweet. Autozone and walmart were alright and the online store had some interesting ones but were either too much or would be a chance that I could buy the wrong size. No matter what, take off the shifter and take it with you so you can compare thread sizes. The universal ones work, but higher chance of screwing up the screws on the knob.
Looking for a Used Sentra in your area?
CarGurus has 50,802 nationwide Sentra listings starting at $1,795.
Search Nissan Sentra Questions
Nissan Sentra Experts
Related Models For Sale