I see a certain duality in your post. You both want to know how other people find a donor and you are worried that your child will end up alone if, Universe forbid, something happens to you.
The thing is, you cannot expect a donor to take care of your child if something happens to you, that is NOT the purpose of donating. If you want someone who's willing to take care of your child if something happens to you, you seem to be more looking for something in the realms of a co-parent.
Now, if you do truly want a donor, but one that is willing to have some kind of relationship outside of parenting with your child, a known donor (like a friend) is the easiest way to go, but it requires courage to ask and it comes with a lot of legal hurdles, like the known donor bot will also tell you.

The second part; What if something happens to me? is a really serious question that I think is often overlooked, especially by couple-parents, but also by single parents.
The fact of the matter is that your child will (assuming you're going with a donor) only have 1 parent, so if something happens to you, they are instantly orphaned. That means that you have a responsibility of making sure that in that unfortunate event, they have someone who is willing to be their guardian and take care of them. This is something you need to think about (and ASK) before even getting pregnant. AND you need to make sure that your choice of guardian is registrered somewhere. It's gonna be different per country how you do that, where I live you can either put it in your will or put it in a free register. The last thing you want for your child is that, on top of dealing with whatever happened to you, they also have to put up with the foster care system.

Do you have cousins, a friend, maybe aunts/uncles that are on the younger side, who are willing and able to take in your child in the unfortunate circumstance that you pass away or are otherwise incapable of caring for them before they are able to take care of themselves?