Take a look at the 1964 catalog and you'll see that Latvia even in 1964 was 10 cents a stamp. Not exactly common even back then--$11.25 per 100, as opposed to Italy 50 cents, Latin American 75 cents, etc. I bought a packet of Baltics at a stamp show for about 10 cents stamp in 2019 or so. I would consider them "uncommon" in that if you buy 1000 stamps on
ebay in a mixed off paper lot, you'd be highly unlucky to get any Latvia or any Baltic State for that matter.
I just did a search for "Latvia off paper" in the stamps category. There are a number of BIN options, 10 stamps for $3, about 25 (some 1941) for $19,a Northern Europe collection for $10, and 173 Latvia duplicates for $79 (not so cheap), a nice 1938 souvenir sheet for $4, etc. That Northern Europe lot is an interesting one but it includes Lithuania, Central Lithuania etc but a nice $10 foray into that part of the world.
Part of the thrill for me, anyway, is the search. What would be the fun if you just bought them all and then you were done :)
I've got a stockbook page of Latvia and Lithuania as part of an auction lot. They're out there, but not as common as you might have thought.
Jackie