Do you mean free synthesizers?

If so then you can check following sites. Choose free version.

http://vstplanet.com/

http://www.kvraudio.com/

http://www.hitsquad.com/smm/cat/PLUGINS_VSTI/

After downloading, you have to put files in the Magix synth folder. Later open Music Maker it should be added. Otherwise you have to do manually from Add vst plugin if in MM 17 you got this option.

Note: it is not necessary that every third party synthesizer vst/vsti plugin will work in Magix. Some time they don't