That's surprising, my local upscale supermarket even has like multiple brands of plain whey protein. I'd probably just order it online then. I like to mix it sometimes with cocoa powder, then just more or less chug it lol
Read the ingredients closely. Sometimes, the sweetener has a chemical name you won't recognize.
Difficulty finding it could just be where you live? Every supplement shop within 50 miles of me (of which there are only 4) has an unflavored, unsweetened protein. And, if I type "unflavored unsweetened whey protein isolate" into amazon, there's 30 bags for me to choose from. I randomly clicked on 5 of the first page results, and none of them had a sweetener on their nutrition facts/ingredient list.