Where are you located?
That can determine what is available.
However, in general, shrimp specific foods are recommended. Fish foods are not the best for shrimp.
Nothing beats fresh food either.
And it doesn't have to be difficult either.
Blanch some zucchini and drop that in the tank. You can cut the ends of a zucchini, which would normally go in the bin, and boil that for 2-3 minutes.
Or, you can blanch some leaves like mulberry, banana, ... any non oily soft leaf.
You can drop in flowers, that you might already have in the garden. As long as you are sure no pesticides or fertilisers have been sprayed on the flowers or leaves.
You can also feed shrimp frozen bloodworms, that is a must to keep them strong leading up to breeding.
Additionally, drop in a few Indian Almond Leaves (IAL or Cattapa). They will breakdown slowly, and help increase biofilm. The shrimp will eat the IAL and biofilm.
Ate some prawns or fish last night? drop a bit of the off cut/ scrap meat into the tank. As long as it is clean of oil or Tartar sauce :)
Shrimps are scavengers, they'll eat almost anything.
If you would like more inspiration of what to feed shrimp, have a read of the Food and Nutrition section of the forum.
https://skfaquatics.com/forum/forum/44-food-nutrition/