There is no way to know which are free in this forum.
But you can bookmark this page.I listed them here.
But those free are not free a lifetime. This due to someone at vbulletin.org complaint to me that I give free updates fro free ones but for paid pre-mades they have to pay for updates even its cheap.
So what I do is, the free is only to the version up to date. If it is for 3.8.1 its only for that version only, after 3.8.2 it will not be free and will be listed for sale.