I mostly use 99designs.com for product illustrations. You can send a description or a sketch to a designer on the platform and get custom graphic design work in return
Review mode lets you drop a comment directly on the designer's mockups—tightening feedback loops.
How to apply it to growth design
Illustration can be a very effective way to explain a new value proposition to a customer. A clear visual diagram can be more quickly absorbed than text alone. And it can really pack a communication punch comparable to the brevity of a searing political cartoon. In its early days, Dropbox became really well known for its illustrations that introduced the mainstream Internet to the newfangled concept of cloud storage.
A quick and affordable way to get value proposition illustrations made is to use a platform like 99Designs. This would be great for graphics that explain "How it Works" or "What We Do." Be sure to get multiple variants created so you can A/B test them.