Jockey and Hanes used to sell men's thongs (mostly at K-Mart). You could also buy them at Target for some time, although, I can't remember the brand. For some reason, that is no longer a thing. K-Mart obviously went belly-up and Target no longer offers them. I think most men don't even know it's an option. Mainstream media still advertises boxers as the underwear worn by most men and occasionally you'll see a cameo for briefs. It's sad, but it is what it is I suppose.
I've gotten far less concerned about wearing thongs as I have gotten older. I still wear thongs or g-strings 99% of the time and if someone notices today, then I'll shrug and brush it off. A decade ago that was not the case. I started MTF almost a decade ago because I felt alone. I was worried I was going to get beat up and shamed in public if it was discovered that I was wearing a thong or g-string. To-date, I've never had an issue. I learned from my friends here that it doesn't matter. I regret having to had to shut down the site 7 years ago, but I am so glad someone brought it back!
I digressed a bit there... To bring it home, I think that to normalize men's thongs, they need to be made available in brick and mortar shops that aren't just specialty shops (like the only place I can buy thongs in-person in Dallas). I think mainstream media and TV-shows, movies, etc. need to stop operating under the assumption that all adult men wear boxers... Boxers suck. I also think that it's important to showcase that more than just gay men wear "alternative" men's underwear.
Honestly, until those things change or we get really lucky on a random fashion week, we're going to remain the minority. Thongs were invented by men for men, but that fact seems to have been lost in history...