They are all safe. Your body kinda tells you when it's not okay. Kinda like sleeping on my stomach, I did it until my body said stop so now I don't do it anymore. As for sex, at the end when you are the biggest it may be hard to do missionary it always was for me so hands and knees always worked...
If not Wal-Mart then maybe Target or K-Mart? We don't have either one of those in my town we have them further off and I've never bought preg. test from there but I know they are a lot like Wal-Mart in some ways. I would try looking again before I give up on them, maybe they were just out...