I thought about buying something like that, but I often work at the house and I'm not rich enough to risk my girlfriend sees me wearing that.

There's an infomercial in Italy for a product called Royal Posture, which is supposed to force you to have a good posture.

I'm looking for ways to do it without having to wear something, though.

There are very similar products on AMZ (USA) for $15-$30 USD.