I always used to think I'd have a baby by the time I was 30. Then, when I hadn't, I though 'well, times have changed; 35!'. Then when I hit 35 it became a more nebulous 'there's still time!'. I think I was about 37 when I started to seriously think about the fact that maybe there wasn't that much more time and perhaps if I wanted to get the job done I should just get on with it myself.
Partly, I did the maths in my head and realised that even if Mr Right were to fall from the sky right that second, I would clearly want to spend at least a year in a relationship with him before trying to conceive. But also I realised I am not particularly someone who wants or needs a partner anyway. It's been quite a long time since I was in a relationship, and I was doing precisely nothing to seek a new one. Being able to embark on this process just taking into account my own views, and the welfare of any potential child, without having to negotiate someone else's feelings as well, feels from my perspective simpler than people having fertility treatment as a couple.
In terms of your dilemmas about a possible future child not having a father figure, and having a different set of experiences to their siblings, I'd really recommend getting some counselling from someone specialist in donor conception. If you go through a clinic, you'll have to have this anyway in order to be able to use donor sperm. You might also find the Donor Conception Network to be a great source of information and support. It might reassure you (and your friends and family) to know that not having a father present isn't inherently something that leads to bad outcomes for children - a child in a stable and loving family, whatever its structure, will generally do fine.
In terms of your options, these days it pretty much comes down to a choice between clinic treatment and private donors. I personally have reservations about the latter as I'm not comfortable with the possible legal and safety pitfalls, but others have used them with success.
I feel like I'm always asking people on here how old they are - sorry! - but, how old are you? I ask because success rates do decline considerably with age, and I always feel that for people with an existing family the the calculations must be a bit tougher. For me, treatment has been E X P E N S I V E but since I've only needed to factor myself in, I could make those choices.
Best of luck to you, whatever you decide!