For unknown reasons, I managed to lose my battery for Jolla and after dissapointing chat with support, it seems it's impossible to get one officially. So, I wonder if anyone has a spare battery or could sell one off bricked device.

Unfortunately, the only way to get one 'official' is to send in the phone for repair, which adds to the total cost. Or, like you try, buy a used one (which might be running out of life, like mine).
Or, try one of the mods for other batterys which also sort of 'works'.

I really hope Jolla is working on enabling buying new batteries.

I visited the repair place. They couldn't help me even if they had a nice stock of the batteries.

is that even legal what Jolla is doing?

Where do you live parasemic? If in germany I could help you.

Like what, provide a crappy service? I don't think there is a law against it. Customers usually vote with their wallets - which, if you think about it, they prety much did in Jolla's case (just look around you and count the Jollas).

Sadly for us, fewer customers results in an ever crappier customer service

selling a phone with removeable battery,
but customers have no chance to get a second one!

strange, very strange.

One point for me not to buy a Jolla...

Solution to Jollas weird battery issue is to support SailfishosOS on Nexus5

The Nexus5 has a 'non-removable' battery but you can replace it by yourself easily

Edit: Wrong copy / paste link.

One of TMO's peculiarities is that search is private, which means you cannot share your search results.
Here is the result you want.

Hope you asked them why, if this was the case, their own website continues to advertise the Jolla as having a "User-replaceable battery"