I do not agree with previous reply until inspecting the broken parts.
The volume switch can be replaced and the cost of the part is usually below $10. However you will need to have the skill to remove and solder the new one.
If being the case of having broken the external plastic slider, then the case is not so simple because you will need to buy a whole case for Nintnedo 2DS to get only that piece.
In any case, it will be advisable if you take it to a repair store (avoid Nintendo, their prices are insane).
Regards,
JM