I've never seen any evidence of too few no-follow links being an issue, has anyone?
No, in fact nofollow links looks like your spamming blogs and forums, etc.
of course, there are tons of nofollow forums for exampleI would not worry too much about the DF/NF ratio, as long as you have some no follow links. A 100%/0% or even 99%1% would be unusual, but 96% is fine, also depending how many RD you have. Is it 10,100,1k RD? If RD is low then the DF/NF ratio could be nearly anywhere.
You can get more no follow links from any source, blog comments, citations, outreach, wikipedia, social media, and more.
but they're not indexed if I am not mistakingEasiest way is to find blogspot blogs and in the comment section build the links these links get instantly approved and finding blogspot blogs based on your niche is not at all difficult.