This is incorrect. The question here is "do you know", the next part needs to be in answer form, currently it's in question form. Should be "do you know where a good restaurant is?"
Yes, either one could be used, but I agree with DoctorP that "Do you know where I could find a good restaurant?" sounds more smooth somehow. Other variations: "Do you know where I could find any good restaurants?" "Are there any good restaurants around here?" "Is there a good restaurant close by?" "Could you tell me where I'd find a good restaurant?"