Where to buy supplies?

So I have a head unit, amp and sub I want installed in my truck. Im getting quotes of around $400 for parts and install, I can't find it in myself to pay that. I got estimates of over $300 in parts alone. Is there any online sites that I can order all of the stuff I need? All I have is the head unit, amplifier (no cables, RCA's) and a sub already in the box. I know I need a harness, the head unit mounting kit, amp cables and RCA's, is there anything else? What's the best place to order this stuff online? Is this a difficult install?

Thanks for the help guys,
Dan

I just recently used crutchfield.com. Very satisfied with customer service and pricing. Speedy delivery as well

Yeah, I like that place too.

Can anyone give me a list of exactly what I need so I can price this out?

I've used both of these sites

http://www.millionbuy.com/

http://www.onlinecarstereo.com/

$300 alone for parts? Way too much. All you need is a dash kit, wire harness, and antenna adapter for the stereo, which shouldn't be more than $50 for pre-03 models. A good brand true awg 4 ga. amp kit shouldn't run you no more than about $120 for everything.
If you have Bose, then the radio harness may be a little higher, but even with that, all the parts shouldn't put you over $200.
A job like this I would only charge about $300, which includes parts and labor. It's a one hour job, two tops.