I’ve rarely done it myself but my roommate did lots and she got lots of things. Fancier grocery stores are usually the best. Depending on the neighborhood they can be locked though. Wealthier areas will have fewer people scrounging so more likely to be unlocked.

If you are a novice, start with non-perishable goods. Perishable goods can sometimes be fine too but requires more judgement.

That said, I have to believe that there must be more services available to you somewhere. Is there some office in your local government that can provide advice?