you can see that the only distinction between a sale reported on a 1099-B or a sale not reported on a 1099-B (your case) is that a sale not reported on a 1099-B will have "Box F" - "Long-term transactions not reported to you on Form 1099-B" checked.
In the desktop version when you start the "Stocks, Mutual Funds, Bonds, Other" interview one of the very first questions asked is "did you receive a 1099-B?" If you tell TurboTax "No" here you're shunted into a slightly different interview that the regular "1099-B" interview, but the interview elicits the exact same information:
What was sold
When was it sold
Proceeds of sale
When it was bought
as you'd get off a 1099-B, and puts the sale into Part II of the Form 8949 with Box F checked.
I assume the online versions of TurboTax also can do this but because I don't use "online" TT I can't give you step by step directions.