It surprises me how anti the suggestion of a sperm donor people are, not just on this thread but generally. It seems such an archaic POV to take, especially given how many women effectively treat men as sperm donors by shagging around indiscriminately, using no contraception etc.
Using a known donor tbh is a legal minefield, and I would strongly advise against.
OP, I understand a little of your position, I lost my parents young (though in my early 20s, not quite so young as you), my grandparents having died when I was in primary school. I knew from when my mum died (I was 21) that I wanted to be a mother, and set myself a target of 30 - if I hadn't met a man by then, and got the happily ever after ending, I would have gone down the donor sperm route. There was no doubt in my mind.
As it turned out, I ended up accidentally pregnant at 25 following a brief relationship. My DS has never met his dad, knows almost nothing about him, and thus far (he is almost 16) this has been a non-issue.
I think you should go ahead asap (I am friends with a lesbian couple a little older than you who have struggled with IVF, and regret not starting the process sooner) and wish you every success with it :)