The renowned journal Arts & Letters out of Georgia College is now, as of yesterday, accepting flash fiction pieces of 500 words or less. You may submit up to three pieces a time through their Submittable page under the subcategory Fiction.
A&L is one of the few college lit mags that pays authors per page printed, and not just contributors copies. This helps to justify the $3 Submittable fee that they charge, and, of course, the fact that they're a world class literary journal.
Have fun submitting, and good luck!