I don't have any experience sketchnoting. But for any freelance business, the answer to "how do I find work" starts with:
- What kind of customers would need this product/service?
- How would those customers look for someone to perform this product/service (or will they have to be educated on sketchnoting first)?
- How can I make myself attract to them?
If I were you, I'd start coming up with a list of situations where sketchnoting could benefit a potential client. That will probably lead you to the type of customer who would buy this service, and hopefully will get you started on the other items as well.
Note: If your potential clients will need to be educated on sketchnoting, then it might not work very well as a part-time business - the client acquisition costs may be too high.