> I have no idea where else I'd look for <recipient country> stamps should I want to include them on a SAE I was sending to said country

I would try to buy them online from their post office. For the USA, there is https://www.usps.com/business/postage-options.htm:

“Print Labels Online with Click-N-Ship

With your free USPS.com account, you can pay for postage and print just one label or a batch of shipping labels online”

Germany has (https://www.iamexpat.de/expat-info/germany-news/deutsche-pos...):

“You simply need to open the app, select the appropriate postage service, tick “Code for labelling” (Code zum Beschriften), and pay with PayPal. You will then immediately receive a code, consisting of the letters #PORTO and an eight-digit string, which you must write in pen in the top right-hand corner of the envelope or postcard. Then, just pop it in the post box, and you’re done! The code is valid for 14 days and can only be used for Germany-bound mail.”

That 14-day limit may not be a good idea for this use case.

The Deutsche Post '#PORTO' method does not apply to international shipments, unfortunately. However, you can still purchase barcode labels online for printing, and conventional stamps are simply values in Euro cents so can be used for both domestic and international deliveries.

In addition, the 14-day limit no longer applies. Deutsche Post were challenged in court, and the digital stamps must now last for as long as conventional stamps do:

https://nrwe.justiz.nrw.de/olgs/koeln/j2023/3_U_148_22_Urtei...

That German app is not available in the App Store in my country (and I presume in any country other than Germany), so I would also be forced to go to eBay for stamps

I just priced a stamp to send to New Zealand using that USPS website and it came to over USD$20 so that's not a realistic option. To be fair I had to take some guesses with weight (what the fuck is an ounce and how many letters fit into one?) and dimensions (they don't have units on that website, so I guess my letter is 6x3 whatevers).

that honestly seems more complicated and likely to fail than just buying the correct stamps on ebay.

You use an International Reply Coupon https://en.m.wikipedia.org/wiki/International_reply_coupon