Hi Micco,
the first thing that comes to my mind is: try their respective websites - it's easy, just type something like
www.roland.eur or www.roland.us or www.yamaha.com or www.ketron.it or www.generalmusic.it
etc.

OR...
do a search on Google using as key words the brand you are looking for (like Yamaha, Technics, etc) and "MP3" or "samples".
You'll very likely get lost in a myriad of sites, but at least Googles lists the most relevant ones first.