Website for gunpartscorp
Midway usa sells tons of model 700 parts. They are one of the biggest sources of general gun parts both online, and catalog sales.
The Remington Society of America has a services/parts link too but e-gunparts.com is a good place to start.
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