Here is a list of free/open source resources for scientific illustration

General (non-science) icons

• Noun Project
• FlatIcon
• UnDraw
• StorySet

Science icons

• Bioicons
• SciDraw
• Servier Medical Art
• IGI Glossary: Lots of CRISPR-related icons
• HealthIcons
• NIH BioArt
• Nitro Bio

Drawing resources

• Making pattern brushes in Adobe Illustrator