Is there such a RAM on Amazon or eBay?

Here: https://www.compuram.de/arbeitsspeicher/hp-compaq/notebook/essential/serie/hp-625/8gb-dual-channel-memory-kit-x3ce5.htm

Yes there's:

https://www.ebay.de/...2564314168

Just normal ram for peptops

https://geizhals.de/...roductlist

https://geizhals.de/...333~256_2x

Yes:

e.g.:

Single module:

https://www.amazon.de/...00QM8FCOS/

The data are:

Memory type: DDR3

Voltage: 1.5V

Version: S0 dimming

Size: 4GB (per module)

Clocking: 1333MHz

However, the low-voltage version with 1.35V is increasingly being offered, so I don't know whether the notebook can process these modules.