I'm one of those improbable folks not based in US. While a tad more difficult to get by these days, I've eventually most always managed to find postcards. All post offices sell stamps (excluding the few that only manage parcels). Also, any postcard sized piece of cardboard that bears a stamp qualifies as a travel worthy postcard – once I did just that, for the fun of it, and it worked. I've once received a stamp beer coaster.

PS. Another fun game is to see what's the most cryptical, vague postal address that does the job. See:

- What is the minimal possible UK address? – https://hkrn.ws/34995370

- Falsehoods programmers believe about addresses – https://hkrn.ws/5791489

National and state park visitor centers often carry them too. Cities and states also have tourist information centers with postcards.

Museums. Bookstores.

Those are the top-two places I see them when traveling abroad from the US.

Museums always have cheap postcards for sale.

i found the usps website has a 61 cent postcard stamp, there is one to choose from