It is to the characteristics of change itself that the mentat-generalist must look. There can be no permanent catalogue of such change, no handbook or manual. You must look at it with as few preconceptions as possible, asking yourself: “Now what is this thing doing?”« (6/6)

Ok. Then public domain or "hang-man" style stick figures. 😁
I've found some very good public domain art while researching mythological creatures, believe it or not.

I would reach out to some new or amateur artists here who do TTRPG work, or something like it. You might be surprised.

There are some good, but less established artists who wouldn't be that expensive. Just factor it into the price of the product.
I imagine it's the major expense in most Kickstarters.

My answer would largely depend on the type of project.

A project that will be distributed or sold to others deserves some professional art, by professional artists.
Something that you are going to use only at your table is a bit more flexible.
There is some good and new public domain out there.