It's known the world over that silk is a high quality, highly coveted luxury product. You should expect that the only place that you're going to find mulberry silk sheets is a high end retailer that's not going to let you leave with a set unless you pay an arm and a leg.
The fact is that mulberry silk sheets are a product that most people aren't going to be able to afford. These sheet sets range between $600 and $1200 per set if you want a legitimate product. You can find a few different sets on this page: http://bedspace.net/best-mulberry-silk-sheets/
The way silk sheets are rated is with momme weight units. This is the traditional way to measure the quality of silk fabric. One momme is equal to 4.340 grams per square meter. 8 mommes is around 1 ounce per square yard. This is also 35 grams per square meter.
The higher the momme weight, the higher quality of the sheet. This is similar to the way thread count has often been associated with the quality of cotton or other types of sheets, only momme weight isn't used in a deceptive manner.
Buying luxury silk sheets means that you're going to have to shop at some expensive online or offline retailers to find what you want. This is stuff that you're not going to ever find at Wal-Mart.