<p>I broke 2 keys (i.e. left ctrl, enter) on my Asus UX31A laptop and it seems that this model is too old to find replacement keys.</p>
<p>Is there any way I can find a 3D model of the needed keys to have them 3D printed?</p>
<p>I do not have a printer nor I have any experience in this field.
What I need is either someone that has models for these or that can point me where I can find such models.</p>

This is more off-topic as an answer, but serves as a possible solution.
Replacementlaptopkeys.com is a resource that appears to have keycaps for the model you've noted.
https://www.replacementlaptopkeys.com/asus-zenbook-ux31a-db71-laptop-keys-replacement-dark-brown-black/
At seven dollars a key, it's going to be less expensive than 3D printing to accomplish your objective.
If you owned a 3D printer, it would not be less expensive to purchase, but the work involved would increase your cost to have such keys commissioned. As a 3D printed object, the strength is going to be less than a keycap purchased from the linked site.