... But although it was so listed in the 1898 Pharmacopeia, it is no longer considered an efficacious treatment for STDs. (But might help for pimples and diaper rash.)
I don't find boric acid as an ingredient in currently common "baby powders" sold in the US, and if included it would likely be in very small concentration.
Most references to its use as a "human medicine" terminate ca. 1890, with a few references into the 1930s. At least one Russian company still includes it in their list of products, but it's doubtful one could get it easily from them.
There apparently is still some use in veterinary medicine, so perhaps your vet, or "farm store" could provide something.
While there may be current uses that I missed with my quick search, and you might be able to get it from your local Rx shop, it's not apparent that there aren't other products considered more effective for most uses now.