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If you're doing any form of print on demand/KDP then Creative Fabrica is what you need.
They have plenty of graphics, fonts & KDP interiors.
They are currently running a sale on their subscription for $10 per month. It's a lifetime deal too, I got it when I first started for $9 per month.
This price is limited to 250 people - after that, it rises again.
You can find it in the discount deal section right here ➡️ https://www.creativefabrica.com/ref/643038/
Hope this helps!