Jerriecan, as Lars says, some freebies can be used for commercial renders, but there are a lot that can’t.
THe readme is where to look on freebies, to check. Even a place like ShareCG, some of the older freebies the usage rights as shown on site, can be misleading as Share have changed the options available a couple of times. I know that the very early ones I put up there, there was no way I could say OK for commercial renders but not for redistirbution, but now I can specify just that with the new options for usage rights they have added. However most of my readme’s contain a phrase which says they can be used for anything but not re-distributed in any form.