You can get it at most stores. It's usually in the OTC pharmacy section, sometimes the "ethnic care" section (i.e., if it's a store in a highly hispanic area it tends to be with the other remedies commonly used in hispanic culture), and sometimes it's behind the pharmacy counter and you have to ask them for it, but I've never had to get a script for it and if some place is crazy enough to want a script then just go on to the next store. I've seen it at Target (the only place I had to go to the counter for it), Walmart, Walgreens, Minyards (the place where it was in the "ethnic care" section), etc... It's usually under $2 for a bottle and, while it's a small bottle, it lasts FOREVER. Just remember that it DOES stain (it usually won't come out of clothes and it will stain your skin for a few days) so if you are cloth diapering make sure to use ones that you don't mind getting stained purple and just be cautious with it. It's not the worst colour to stain things, kinda pretty in fact, but it's not exactly something you want all over everything.
You could "outline" baby's diaper area with vaseline to help, but I would still be cautious.