You should try to get a copy of the 1099-B. However, it is not required to enter the stock sale and associated capital gains. Use the instructions in the link below and when asked if you received a 1099-B, answer NO.
Form 1099-B (Proceeds from Broker and Barter Exchange Transactions) reports the sale of stocks, bonds, mutual funds, and other securities. Here's where you enter or import your 1099-B (even if you did not get one) in TurboTax:
box category C or F on Form 8949 is for stock transactions for which you don't have a 1099-B.
This is not common. Losing your copy doesn't count as no 1099-B since they are also supplied to the IRS.