Although you have to ask your Tourney Organizer, the Coin Flip App is generally NOT allowed, as are any electronical devices.
Dice and coins are the only widely allowed ones.
Not all coins are allowed. Only coins that have been released in preconstructed decks, blister packs or other PUI released product after Holon Phantoms are allowed in tournament play. So the HP ON rule applies to coins too. And they have to bee in good condition.
As for dice, d6, d12 and d20 are the most preferred ones. d4 and d10 are not allowed at all. As a tournament organizer and head judge in many tournaments myself, I strongly recommend a d12
Check from your tournament organizer or head judge to be sure if you have a legal randomizer.