I am not sure if this is the best. But, YC (Y Combinator) provides some resources about startups. You can check them out at https://www.ycombinator.com/library.
If you’re in an early stage and need a quick logo, check out thi hassle-free and fast logo generator: https://amee.la
Gives you a quick mark so you can keep focussing on building something people want.