I've found all of the singles at two Minnesota K-Marts. I wish I'd known they had them when I was out east this week. I checked Wal-Marts, Targets, and Toys R Us stores in South Carolina, North Carolina, Virginia, and DC with no luck.
The cool thing is that most of the TRU that I hit had Comic pack 44 and Viper pits. My gas mask army is quite a bit larger now.