It depends what you consider cheap, but:
Hang's Happy Mart on Linh Lang St, opposite Linh Lang Market. Linh Lang St is off Dao Tan, about halfway down. The Market is also good for quality fresh veg, but the fruit tends to be overpriced.
Dan's Shop at Van Phuc and also on Linh Lang St (towards the Dao Tan end).
They're a couple of small one's in the little alleyways off Dao Tan, near the Danly Hotel, but I can't be any more specific than that I'm afraid.
