OK - I just spoke with someone at Customer Service. They had me on hold for a while and then gave me to someone at sales. They had me on hold for a while and then said they couldn't help me because they only had the full systems, not just parts. They then told me to call 800-326-1688, press option 3 and then press option 1. I was then asked to leave a message for my RMA. RMA!? My system is out of warranty, and I don't want something repaired or to return any merchandise, I just want to buy a new remote.
So I hung up and tried calling back again to try different options. Then I got a "We're sorry, we are now closed."
Anyway, I fixed my old remote tonight. It's been about 10 years since I've soldered any electronics and I had to buy a soldering iron just to do it, but it's working now. I was still going to buy another new one as a backup, but since it's such a hassle, never mind that. I think I'll buy a learning remote off Amazon.com and train it to speak DSM-520 while my official (now repaired) remote is working.
Thanks, D-Link!