There are multiple places you can get free SAT preparation resources. Some of them include the following:
Google Books: you can read the official SAT prep book published by College Board here, also the ones published by Barron, Princeton Review etc. If unable to find, you can simply view the books here.
Khan Academy: This one is quite obvious, but one of the most used ones. You can view free study videos and give practice tests.
Collegeboard website: CB itself publishes some free resources on its website, and they are the ones closest to the official SAT test.
CrackSAT: this website contains a treasure trove of SAT papers.
Torrents: this one is actually illegal, so proceed with caution (use a VPN preferably). You can easily get free ebooks, past papers and preparation videos, many of which are actually available at a cost otherwise, for free on torrents.