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.
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Korg Kronos 61 and PA3X-Pro76, Roland G-70, BK7-m and Integra 7, Casio PX-5S, Fender Stratocaster with Fralin pickups, Fender Stratocaster with Kinman pickups, vintage Gibson SG standard.