That's a pretty niche bit you're after!
Honestly, I don't know that you're gonna find them. You might try Zinge Industries. I know that bloke tends to do those
"I didn't know I needed this until I saw it" pieces. He might have done pad locks at some point. It's been a while since i've checked his site.
The other "easy" option would be to look at some of the space marine models. Ones like librarians and chaplains or grey knights. Those guys who have a tendancy to carry books around. Some of them are bound to have locks. Although the viability of this option is tenuous at best.
In reality though, your best bet is to probably knock em out yourself. If you've any skill with modelling putty then wee locks shouldn't be excessively hard. You could also maybe cut up little chunks of sprue to resemble locks. Drilling little key holes and all that.