Costco is now selling platinum bars. Here's what to know, from price to availability.

Costco is expanding its precious metals offerings, building on top of its rollout last year of gold bars, which often sold out within hours. The retailer is now adding 1-ounce platinum bars to its lineup. 

Described as 999.5 pure platinum, the new bar is Swiss-made by a refiner that has earned the Good Delivery accreditation from the London Bullion Market Association (LBMA), an international trade association for the precious metals industry.

Costco's gold bars were such a hit with customers that the retailer had trouble keeping them in stock. After the retailer added silver coins to its lineup, Wells Fargo estimated that the chain was selling as much as $200 million of gold and silver each month to its members.