The Yamaha batteries have the dubious distinction of being slightly dearer than the Bosch ones.
The ever reliable Bike Discount has one for £668.
At least with Bike Discount, if they say it's in stock, it is, so you can be confident of getting the item within a few days.
For what it's worth, the batteries do seem to last well.
I have 'original' Bosch batteries which are still holding all their charge after five years.
Some spare batteries for the cheaper Chinese ebikes are nearer £300, but some of them seem to conk out after two or three years, so overall Bosch/Yamaha comes to about the same price.
My Bosch ones may even work out cheaper, if I get another a year or two out of them.
I'm not aware of any 'pattern' batteries for Yamaha, but they may be a false economy even if you could find one.
The systems are locked down, so a pattern battery may simply not work, or it will be made from cheaper cells which will wear out after a few years.
Factory original is best.
https://www.bike-discount.de/en/buy...-13.8ah-frame-type-battery-631259/wg_id-17900