Can not find the replacement battery

This is the community repair forum and isn't a great choice for contacting iFixit Store personnel. Your best bet is to email them at support@ifixit.com.

That being said, they buy their Pixel parts directly from Google, so if they don't have any listed in the Store, my guess would be that they're no longer available from Google.

I looked around and I can't find anyone selling Genuine Google batteries for your phone; however virtually all of the ones I did find are labelled "OEM" and look like exactly the same part with the same part number. Here are a couple that should work.

For Google Pixel 1St Generation Replacement Battery B2PW4100 + Tools 791348932015

Google Pixel Battery Replacement Kit with Special Installation Tools, Extended Life Battery, Video Instructions, and Full One Year Warranty - Walmart.com

Only thing I can suggest is that you be very careful removing the screen; apparently they're pretty fragile and it looks like it's easy to get between the digitizer and the LCD rather than behind the LCD. If you do that, you'll destroy the screen.