Had that problem and what came to mind was ebay at first.
Check-out car-stuff.com
That is where I ordered my shock absorber.
They sell after market
Nissan Parts there and for shipping, but you'll have to shoulder state taxes though.
I'm sorry for just dropping a site name. Will post this direct link to on car-stuff where you will be directed to the altima page instead of querying various info.
Thanks