If you're planning to buy gold at Costco's warehouse stores, you may need a backup plan. Here's where else to look.

There are plenty of items to load up in your shopping cart at Costco -- but gold bars shouldn't be one of them.

Costco appears to have found a revenue mine with sales of gold bars, as a survey found that some warehouses are unable to keep them on shelves. Bloomberg found that 77% of the 101 stores it surveyed in the first week of October had sold out of the bars, the financial news outlet reported Monday.

Retailers like Costco and Walmart are selling precious metals including platinum bars, driving a new demand for precious metals.

UBS estimates exclusive releases of the pop star’s merchandise could boost the retailer’s fourth-quarter revenue by about $100 million.