What are some alternatives?
When comparing Illustrio and Fontello, you can also consider the following products
Evie by unDraw - MIT licensed illustrations for your next project
Flaticon - A database of free vector icons.
DrawKit - MIT licensed SVG illustrations, in 2 styles
Font Awesome - Font Awesome makes it easy to add vector icons and social logos to your website. And version 5 is redesigned and built from the ground up!
IcoMoon - IcoMoon is an icon management tool that is build to provide iconography.
IconPharm - IconPharm is a library of open source icons with recoloring and resizing tools.