Where is good for Li-iin appliance batties?

It’s not financially viable to pay for a genuine replacement battery, cheaper to buy a new item. Similarly, I don’t want a cheap knock off that will burn the house down while I sleep.

Where do I get reliable replacement batteries at a fair price?

You’ve listed the two options – pick your fave.

What’s it for, and also, have you seen the charging thread…?

No, I’ve not seen the charging thread.

It’s for a robot vacuum cleaner

If the pack is basically goosed and you can crack it open and think you could effectively stick it back together, and you can solder, I’d have a go replacing all the cells with something supplied by RS or similar, but TBH if you’re able to do all that you probably know enough that you don’t need that advice from me!

As far as the manufacturer’s price goes they’re probably prepared to stand by the quality of the cells they fit, rather than using reclaimed crap from any old source that might burn your house down soon as look at you.

I’ve no doubt the manufacturers battery is a solid option, but it’s cheaper to send the broken one to landfill and buy new.

Such is life I’m afraid. Some businesses prioritise new unit sales over repairability. That can inform consumer’s choice as much as price.

Having said that I’ve bought stuff in the past with promised repairability (E13 dropper, white industries freewheels to pick on two in the bike industry) where spares were either unavailable, prohibitively expensive, or meant vendors were breaking down complete units to provide spares.

On the flip side, hope hubs and their other products generally, brand x droppers. Sometimes it works.