How to come up with good ideas?

How to get startup ideas - Paul Graham (Y Combinator), 2012
Schlep Blindness - Paul Graham (Y Combinator), 2012
What you'll wish you'd known - Paul Graham (Y Combinator), 2005
How to Come Up with Profitable Online Business Ideas - Channing Allen (Indie Hackers), 2020

How to iterate to get product market fit?

How Superhuman Built an Engine to Find Product/Market Fit - Rahul Vohra (Superhuman), 2019

Books

The Mom Test: How to talk to customers & learn if your business is a good idea when everyone is lying to you - Rob Fitzpatrick (2016)
Inspired: How to Create Tech Products Customers Love - Marty Cagan (2017)
Start Small, Stay Small: A Developer's Guide to Launching a Startup - Rob Walling (2010)

Where to find some problems to solve?

Upwork
Ebay Community - Tools & Apps
Amazon Seller Forum - 3rd party apps