Laptop's internal keyboard as an external one?

You just have to ask for it and a good computer shop will simply order it from their distributor.

Here is a catalog of most Acer Laptop Keyboards, please note that this is not a store but rather an extensive catalog, they share the same product code/name as the manufacturers/distributors.

To make the process easier, look for a product code at the bottom of your laptop keyboard.

This keyboard on icecat is compatible with Aspire 4736G/4736Z Series, product code: KB.INT00.261.

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Instructions from the service guide hosted on icecat: (Remove power connector and power first!):

There are controller circuits that will convert a laptop keyboard to usb, i'm currently using one. I'm not sure where you can get them but they obviously exist. They are used to create keyboards for tablets.

Doing some research, it turns out that laptop keyboards dont have the controller circuit. They are just the keys with the contacts, and that ribbon cable connects those contacts to the controller circuits which is inside the laptop.
So there is no way to convert that ribbon cable alone to a PS/2 interface. You need first the controller circuit, and only then you can think about converting it to a PS/2 plug.

And depending on how the keyboard's wire matrix is, the ribbon cable will have more or less contacts which match those on the controller. So you either need the exact same controller for the keyboard you want, or you can get a controller from a spare keyboard and manage to rewire the connections to the controller so they match correctly.