Oh, I know some mushroom farms will just kind of give away their used growing media if you ask. Another resource to check for.

Some municipalities will have programs where you can get free or low cost compost; I’d ask around there, as well as look for local gardening clubs and mutual aid groups to see if anyone has some to spare.

Also, both rabbit and Guinea pig droppings can be pretty good soil additions, if you know anyone who keeps either as companions/fosters or a rescue organization for either.

Also also, if you have any friends who keep fish, the water left over from when they change the water out in their tanks is also nutrient rich