Little did I know, there are actually four singles available, with three different "official cheer songs" (one for each year the team has existed), and some other songs as well ("Rakuten Eagarus! Go go go!"). I suspect what I'm looking for is the official 2007 song, but it's hard to be sure. So, as long as I'm being Mr. Crazypants, might as well collect the whole set.
So, I guess this is going to cost me at least 4100円 (for the singles alone). Plus shipping. Which probably won't be through Amazon.co.jp, because that would apparently add up to 8000円 for them to ship it to me -- maybe I'll see about ordering it and having it delivered to a friend over there.