Do you need the original manual?

You could check archive.org

There is also a GameFAQs page.

Edit: think i found one for the NES

https://gamemanual.midnightmeattrain.com/entry/%E5%BF%AB%E5%82%91%E3%83%A4%E3%83%B3%E3%83%81%E3%83%A3%E4%B8%B8

https://www.ebay.com/itm/256273647337

Ebay has a copy. Super pricey. Wasn't able to find a digital version

https://www.ebay.com/itm/256273647337

Found one. Took me 10 seconds. Super expensive though.

Here are some pages from an auction site, you may be able to do reverse image search to find more.

https://xzoanujr.tajsuave.click/index.php?main_page=product_info&cPath=654_655_663_1082&products_id=249442

https://static.mercdn.net/item/detail/orig/photos/m61700421783_6.jpg

https://static.mercdn.net/item/detail/orig/photos/m61700421783_7.jpg

https://static.mercdn.net/item/detail/orig/photos/m61700421783_8.jpg

No luck here. I tried searching in Japanese too but it's not present in any gameboy manual archive or archival site I knew about or DDG.

I see it on mercari. If people could fund it, I can buy it since I live in Japan and upload scans.

The internet archive is my usual go to. I didn’t have any luck but if someone finds a scan of the manual I’m sure the archive would be a good place to preserve it.