Who knows, mate. There are so many idiosyncrasies introduced by the early games that the later ones had to grapple with.

In my mind, Machoke which live around humans voluntarily wear the brace, which keeps their strength under control so as not to accidentally harm them, but I'm open to the idea that Machoke further afield wouldn't have them. As to where they could get the belt, it may be distributed through Pokecentres.

edit: fwiw I think the reason the whole 'Pokemon (base versions) don't wear clothes' thing came about because of Gardevoir, where people thought Gardy was wearing a dress? Then they had to say, no, actually, it's part of the Pokemon, but unfortunately people retroactively applied that to Pokemon which do adorn themselves in clothing or other items. Like, you won't find people retrofitting that to say Farfetch'd's (damn that looks awkward) leek is part of it.