For example I wrote a course on building a SAAS app with Flask. It's available at: https://buildasaasappwithflask.com/
It covers everything about user registration, profiles, subscription billing, 1 time billing, invoicing, and about 50 other things you would likely want to do in a SAAS app or any application really.
The course comes with the source code along with 15+ hours of video explaining every line of code in stages, life time free updates and life time support for close to half the price of what Spark charges just for the source code for 1 site license (with the Flask course you can use the code in however many projects you want).
Spark's business model seems interesting though. I don't use it personally but do you just get the source code and nothing else? How do they limit you to 1 site if you end up with a local copy of the scaffolding / code base?