With 158,000 miles and plans for 10,000 miles worth of cross country driving, I thought I would be proactive and replace the brushes in my alternator, so I didn't have to do it in the middle of nowhere.

The brushes are part of the regulator. You can't buy them separately. Rock Auto offers a regulator for $119, which is not much less than an exchange alternator. But, I can across Maniac Electric Motors, in Dallas, that sells a rebuild kit, consisting of a regulator and both alternator bearings for 1999 - 2004 for $47.
http://www.maniacelectricmotors.com/alternator-rebuild-kits-2000-2004-volvo-c70.html

Before ordering, I wanted to make sure that I could actually get the regulator out, so I pulled my alternator and discovered that once you have the alternator our, removing the regulator is easy (3 screws and 2 nuts). When I got my regulator out, I found that I still had 35% - 40% of the brushes remaining, even after 158,000 miles, so I put it back together and reinstalled the alternator.

If you have the skills to get the alternator out, you have the skills to replace the regulator. Check youtube for videos on how to get the alternator out. It's not easy. Different procedure for turbo & non-turbo. It may be necessary to remove the upper radiator hose on the turbo. I was able to squeeze the alternator past the hose on my 1999 T5, but newer models may not be so lucky.