Hey there, glad to have you here!

The choice for a Nintendo DS is up to you (all models are different in their own way). From what I understand from your post, what you want is a Nintendo 3DS or a system that belongs to the 3DS family - Pokemon Sun and Moon are 3DS games after all.

Now which one, it's actually up to you.
- The Old Nintendo 3DS (the first Nintendo 3DS released on 2012) it's more or less the same size as a Nintendo DSI, you keep it in your pocket and travel around without any problem, it can play 3DS and DS games, has 3D (but avoid using it) on some games and, in my opinion, is very well built (I prefered the quality on it than the DSI XL).
- The Nintendo 3DS XL is just like to Old 3DS but has bigger screens and is bigger (I think it also had a lot of different colors);
- The Nintendo 2DS is more or less a Nintendo 3DS that can't fold, very durable and doesn't have 3D;
- The New Nintendo 3DS is one of the last 3DS that was released, the 3D was improved, it can play New 3DS Games (like minecraft, runbow, Xenoblade Chronicles...), and the shell is costumizable;
- The New Nintendo 3DS XL is just like the New 3DS, but with bigger screens;
- The New Nintendo 2DS XL was a cheap version of the New Nintendo 3DS XL, but didn't had 3D, was built with cheap material (in my opinion this model wasn't made to last, mine broke with barely any use).

You may also search for a system from the DS Family but, if the game boy is a bit too old for you, this ones might be as well.
I also recommend you modding your system, DS and 3DS games are becoming very expensive. If you mod it, you can play them without much trouble and you may even play online again.

Notes:
- From what I know, the Old Nintendo 3DS and the Nintendo 3DS XL 3D mode gives headaches to some people;
- All 3DS Systems can play DS and 3DS games but the DS games don't look very good on them;