[Eridanus]

Bettas with the marbled gene Will have those colours change all the time

Ill second that!
Full marbles will change often, and drastically usually.

Koi Bettas do have the marble gene but most aren't as drastic. Not like a complete colour change such as from blue to red but more often pattern shifting or adding a new colour to the pattern. (in my experience!) For example, I had one kid that when I got him was pale yellow and white. A few months later he was a bright canary yellow with black and Hellboy pattern and blue iridescent patches.

 

[Eridanus]

if I may ask, where can I get a good koi betta?

Sure!
Some good online sources are Aquabid and eBay. Though most of the breeders on Aquabid are from overseas so if you don't want to work out transhipping and all eBay might be better.
Personally, I've gotten my koi Bettas from pet stores in my area.
So, just depends on what you're wanting to do!