You could create a list containing the numbers 0 to 8000.
Then looping 1000 times generate a random number between 0 and the length of the list.
Remove that element from the list and add it to an output list.
By removing the element you ensure that your selections are unique.
while (outputList.Count < 1000)
{
index = random.Next(0, inputList.Count);
outputList.Add(inputList[index]);
inputList.RemoveAt(index);
}