The most I've seen in one sitting is from the doctor in Goodsprings. I've seen hom carry 12 or so. Other vendors don't seem to sell more than 3-5 at a time and raiding first aid boxes is extremely hit or miss, but always worth it since it saves caps. I pretty much just go into every building I can find and take/steal stuff from first aid boxes. Sometimes it's tricky with a ton of people around (who will foil your thievery) but they usually clear out at some point. As such, I think finding them is the best bet since there are seemingly tons of buildings.
"Tadakatsu Honda shall make orphans of your souls"
New Vegas Medical Clinic, the doc there will be carrying 10-25 at a time. Just buy them all, step back and wait 3 days, then buy them again. She re-supplies on doctors bags and super stimpacks each time too. I like to go between there and gun runners when Im re supplying on meds and ammo.
According to my folks, I'm Russian, Scottish and Polish. Which means I can drink heavily, can't tan, and occasionally put screen doors on submarines-Seth Green