I know there is とある~ but I don’t remember it ever actually being used in spoken speech…
So, how you you say “a certain X” or “some particular X” using normal japanese?
Also, more importantly, how do you say the same thing about people? For example “some people do X” or “a certain person does X”?
For “a certain person”, I’ve seen ある人. So perhaps something like ある人はXをする for “a certain person does X”。But while I can’t think of anything, I feel “some people” would be written differently.
As for “a certain X”, I recently learned the expression ある程度 (ていど), which is translated to “to a certain extent”. Perhaps ある is used often to mean “a certain”, but this is just a guess.
I know there is とある~ (ある) but I don’t remember it ever actually being used in spoken speech…
(と)ある is normally used in writing. In spoken language, if you want to say, for example, “certain girl was drinking tea” you simply say 「女の子がお茶を飲んでいた。」.
There is no need for anything more.
I hope it helps,
Cheers 
You can use 某 in front of words. You see it sometimes when people aren’t allowed to refer to the specific thing e.g. 某会社 . I’ve heard it occasionally on radio shows with rules against refering to specific companies or products I guess maybe for legal reasons. Something like " I was playing a certain game about a plumber" rather than Mario. Can also just be vagueness though.
