As a preposition, "by" can indicate the means of achieving something
So in this case
Ordering my food by delivery
would be most accurate. Especially since the context of the sentence is the difficulty in ordering food given the extant epidemic.
"In" is inaccurate since you're not ordering food in delivery. You might have ordered food that is currently in delivery (As in the state of being delivered), but you are not ordering food in delivery.
"for" indicates purpose. So if you're ordering for delivery, it implies that the food is intended to be delivered. This can be correct, but it's more appropriate from the restaurant's perspective rather than from the perspective of the person ordering the food IMO.