There are loads of online magazines that published flash fictions, depending on places between 300 to 1000 words. A lot of them have a submission page on submittable so it's a good place to start. You can do a search for "flash fiction". Others at the top of my head that are not on submittable:

Barren Magazine, Spelk Fiction, Fictive Dreams, Fiction Pool, TSS Publishing, Jellyfish Review, etc...

Also there are quite a few running flash fiction competitions such as Bath Flash Faction Award, Reflex Fiction quarterly competitions.

It also means that you can have a read of their back issues or previous winning stories to see what kind of flash fiction they are after.

Good luck!