The free version is available here:
https://www.malwarebytes.org/antimalware/
However, when installing it, it's IMPORTANT to remove the tick alongside the option to take a free trial of the Premium version. If you fail to do so you'll end up getting prompted for payment.
If you've ALREADY installed the free trial of the Premium version (because you failed to remove the tick) you should remove it completely by running mbam-clean.exe (rather than from your control panel), RESTART YOUR COMPUTER and then install the (genuinely) free version by remembering to remove that tick.
Note that (unlike the Premium version) the free version doesn't 'run in the background'. It only 'runs on demand'. So you need to remember to actually run it occasionally (perhaps once per fortnight) otherwise it will do absolutely nothing.