To get just a few spare parts, Numrich is an excellent source. I don't like paying $5 in shipping for a $4 part and if you call their 1-800 number during business hours, they will provide a $4.95 shipping credit code to be used for online orders. They also have shipping coupons on their Facebook page on Wednesdays.
Another good source is the vendors on Ebay based in the Czech and Slovak republics. Their prices on single items are excellent but shipping is very steep. I emailed a couple of them and they were accommodating with combining shipping on multiple parts. Much of their inventory is new-old stock.
A couple of years ago the best parts kit deal was to just get another $399 VZ2008. Clearview Investments and Centerfire Systems still sell kits for about $300.
I just saw this thread from 2007. Scroll down a bit and you'll see a listing of different parts kits. Dont cry when you see the prices:
http://www.weaponeer.net/forum/forum_posts.asp?TID=3540&PN=21&TPN=3