IF you're looking for a stock front bumper and/or sunvisor, your best bet is a Nissan dealer, unless you are looking for used parts. Try:
Discounted Nissan Parts
Lower ball joints and timing belt can also be picked up at a Nissan dealer, like the one mentioned above. It's tough to beat the quality of genuine Nissan parts. If you go aftermarket, stick with name brands, like TRW, Moog and Gates. Gates makes a "timing belt componant kit" that gives you the timing belt and tensioner, something I would recommend replacing. Obviously, you can get the tensioner seperately from Nissan and aftermarket sources. Other things to consider when doing the timing belt: water pump, front cam & crank seals, drive belts. I'm not a big fan of aftermarket drive belts as they are usually just a little "off" the factory size. Nissan belts are usually made by Bando or Mitsuboshi. I would recommend Bando belts if you go aftermarket or just get the genuine Nissan belts. Nissan seals are usually inexpensive. I would recommend a new water pump, if you replace it, as opposed to "reman." water pumps. I also think genuine Nissan coolant is superior to many of the aftermarket brands, although it is a little pricey.
For aftermarket parts, I usually deal with these companies:
Discount Auto parts Low Priced Auto Parts Wholesale Auto Parts
Discount Auto Parts
As far as the stereo, assuming you want to go aftermarket, I really like
http://www.crutchfield.com/
I've dealt with them for about 10 years. Their prices are fair, selection is great, customer service is excellant. They give lifetime tech support, supply installation instructions and harness adapters and they are very good at dealing with warranty returns.