Try going out into the burbs. Germantown is usually a good bet. There's a joint called the Park Place that usually fills up last for Memphis in May, another huge Mempho event. It's got some issues - loud housekeeping staff and the like - but the hotel itself is lovely and usually is around $100 a night.

If you're really desperate and Tunica is full, try Jackson, TN. It's maybe an hour and a half drive but it WILL have a ton of hotels, most likely.