I have an Aztek single action siphon feed airbrush and used a gravity feed cup with it all the time. Mine came with one, but I have seen them for sale around. You can use just about anything that will hold paint, so long as it fits in the hole in the side of the airbrush.
Your best bet however, would be to save your pennies for a few weeks and buy a good double action gravity feed brush from dixieart or bear air. You'll find any quality brush well worth the money. I had an Aztek like you, but I saved up and got an Iwata Revolution for 59 bucks, cheaper than my Aztek was, and almost infinitely better. A pasche or badger would do just as nicely, I can't reccomend that enough.
Madda
Trifles make perfection, but perfection is no trifle. -- Leonardo Da Vinci
Tact is for those who lack the wit for sarcasm.--maddafinga