Danish,
Ask Petsmart when the usually get crickets delivered to the store. They usually get really small 'small' crickets on delivery day so they stay small through the week while they sell them.
Josh's Frogs has them on their website for sale in bulk 500 - 1000. ReadyReptileFeeders has them, but I have not ordered from them yet. It ends up being a tad cheaper to buy them in bulk, but the shipping will kill you.
You frogs will not eat any pre killed food. They need the movement to trigger their feeding instincts. They will eat fruit flies, but you will need to have living ones for them to eat.
Dust your flies or crickets with supplements to help your frogs be healthy. 3 times a week with Calcium with D3 and one time a week with a multivitamin.