Remington, gun shop, e-gunparts.com
Website for gunpartscorp
The Remington Society of America has a services/parts link too but e-gunparts.com is a good place to start.
Midway usa sells tons of model 700 parts. They are one of the biggest sources of general gun parts both online, and catalog sales.
I would start with the numrich gun parts corp.They can be found on the web.
Some parts at Gunparts.com can be found. Mags are hard to get.
There are 3 options,,call Remington and ask, they do not have it posted on there web site but might have it around. Buy it---http://www.gungarage.com/remingtonacc.htm Go to a gun dealer and ask, most have one for repair parts.
Check the service and parts link on the Remingon Society of America (A collectors organization) http://remingtonsociety.com/rsa/service
Find an authorized Remington Repair center. You can find a list on Remington's web site.
Depends entirely on what your looking for. Personally I think your better off grabbing a 700 ADL for a few bucks more, the 770 is a lower cost rifle and you get what you pay for. Not nearly a tight and smooth and can rattle a bit ;)
Wal-mart
Try eBay or gunbroker.com
That's a really good question, they are hard to come buy without just buying a clunker for parts but get a matching model/type they are not all the same. Watch gunbroker.com, I think gunsamerica.com also has parts maybe gunsinternational.com and auctionarms.com. You might also try the forums on remingtonsociety.com