I'm going to make the assumption that you haven't yet identified people you would like to mentor you, in which case I'd start there.
How I've personally done it is by first determining what kind of advice/skills I am trying to get better at. For instance, if my company is in B2B and sales is not my strong suit, finding a mentor in a similar space that I'm in but has at least 5-10 more years of experience than I do has been incredibly helpful.
For you, as someone who's a solopreneur, I reckon that it would probably be very useful for you to find other successful solopreneurs, starting to find someone maybe 2-5 years out from where you are now, and then working your way up to someone with more success and experience.
Once you've identified such people, there are different ways of trying to approach these people, depending on how difficult they are to reach. Some ways that have worked for me are to get connected with someone via an intro from a trusted friend, family member, professor or mentor. Other times, I've reached out to them directly telling them why I was interested in connecting with them and seeing if there are ways I can provide value to them. This approach may take more time, but this many people are likely busy and not able to help until they trust you. Hope that's helpful!