I always find atari game carts at GOODWILL & THE SALVATION ARMY and at some yard/garage sales too.
I even found an Atari 800 in great shape for $20 and it works just fine however I did have to buy a power supply for it.
I used to live in Cleveland and there was a store that had TONS & TONS of atari carts 8 bit, 2600, 5200, 7800.
The store was called Tomorrows Heroes