Can I use other brands of window cling in a cricut? Say I have some non-Cricut window cling and a Cricut Explore.  Cricut also sells their own window cling, but I have none.

Is there any harm in using non-Cricut brand materials for a project?
Certainly you can.

Cling material is vinyl and there is nothing special about the product on which Cricut puts its name.

If you go too far afield in acquiring non-brand name supplies you might want to pay attention to the thickness and adjust the material setting for your machine accordingly.