Hi Twicken YT,
The good news is that it is very possible to replace the screen on your phone by yourself. You'll need a couple of tools but not too many and you can get them all here on iFixit if you want. Sorry to say that they don't have a guide to your particular repair at this time; there are just too many devices out there in the world for them to be able to cover everything. However, I did find a Spanish site that has just what you need. Here's a link to their guide.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/samsung-4/samsung-galaxy-a12-nacho-1361/pantalla-completa-12291|Manuals / Samsung Galaxy A12 Nacho / Full screen|new_window=true]
As far as locating a replacement screen, it's going to depend a lot on where in the world you're located, but you can start by checking the big retail sites, Amazon, eBay and AliExpress; they are all likely to have the screen and may be able to ship to you.
Good luck and be sure to let us know how it goes!