Making your own is the best way. IF you want cheap stuff you can also go to the dollar tree for plastic and wicker baskets, cat toys, dog ropes, baby rings, construction paper ect Binder clips have so many uses here. To hold down a blanket, to hold some pieces of construction paper for them to tear off and use for nesting materials, I'll attach popsical sticks to the side of the cage for a removal challenge.
Turn your old clothes into hammocks-a jean hammock or a tshirt to snuggle in. Fleece is cheap and you don't have to sew it. I personally like grommets so I don't have to sew and have an easy time hanging them.
Cardboard boxes-like shoe boxes, tissue boxes, pop boxes-all good chewable cubbies.
If you have untreated safe treats in your yards, use the trimmed branches for climbing and chew items.