There were a few on sale at YHJ immidiately after the 23rd, but I can't find any Suigintou's there anymore with a quick glance, only the Shinku and Hinaichigo kits.
Yakult is quite real and I even drink them occasionally. The joke originates from an episode of Rozen Maiden where Suigintou appears infront of Souseiseki's master and asks him wheather he has his fill of Lactobacillus drinks. Apparently, the Japanese fans liked this joke and turned Suigintou into an unofficial mascot for Yakult :).