Is there a resource out there some place that has custom UITabBarSystemItem icons that others can use. I know you get some from Apple to start, but I would imagine there are a lot more that are very common. I don't see a reason for each person to recreate the wheel for things like home, settings, etc...
You can get some free icons from Glyphish under the Creative Commons Attribution license.
Also, although it's not free there's a nice icon set available from eddit. These work fine for TabBar items.
Check out IconFinder. They have a very robust collection, and you can sort by image size. though these are not always perfect for the TabBar, you can find some good generic icons. You can also filter by license to find icons that are available for commercial use.
What would a line of code look like to use 1 of the built-in icons as 1 of my tab-bar icons?
You can try:
UITabBarItem *anItem = [[UITabBarItem alloc]initWithTabBarSystemItem:UITabBarSystemItemMore tag:0];
And the system icons are described and shown in: http://developer.apple.com/iphone/library/documentation/UIKit/Reference/UITabBarItem_Class/Reference/Reference.html