EDIT: It's been brought to our attention that, aside from traditional Pajamas, which - according to info collected below - cannot be found anywhere in Fallout 76, short of those sold via the Atom Shop, the Bathrobe may be a suitable replacement.
Fallout 76 Bathrobe is an item sold by Flauresca, the Apparel Vendor at Whitespring Resort, and it, too, qualifies as Sleepwear for Challenge purposes. Big thanks to readers for pointing this one out!
Where to find Sleepwear in Fallout 76?
ORIGINAL STORY: Fallout 76 has got a major problem: there's just not enough pajamas to go around! If you've been trying to complete whatever Weekly and Daily challenges the game throws at you, you've probably seen that you need Fallout 76 Sleepwear to complete some of them, but we've got bad news.
Following a rather thorough bit of research, we've come to terms with the fact that you might have a fair bit of trouble finding Sleepwear anywhere in the game, and that's because it straight-up doesn't spawn anywhere at all.
The only place where you can - according to the latest information - find Sleepwear / Pajamas in Fallout 76 is the infamous Atom Store, and they're not even cheap! Getting a set of Pajamas will set you back a hefty 600 Atoms, which is roughly equivalent to $6!
To call this cheeky would be an understatement, but that's just how it is - the only way to get Pajamas in Fallout 76 is to straight-up buy them with premium currency. Sad as that might be, the good news is that if you're in need of Fallout 76 Sleepwear for the purpose of completing the 'take photos near Vault 76' challenge, you only need to complete 5 out of 7 offered challenge options, so feel free to ignore the Sleepwear one entirely.
Whether Pajamas will ever turn up as a drop in Fallout 76 - we do not know, but stay tuned for more information on this particular bit of info.