Place list of desired numbers in column. You can do this by typing in the first two numbers in successive cells then dragging the cells down until the desired number. For example type 500 in B1 and 505 in B2 then highlight both cells and place your mouse cursor over the bottom right corner of cell B2 and drag down. This will give you a list of the numbers.
Now in another cell you can use the following formula:
=INDEX($B$1:$B$51,RANDBETWEEN(1,COUNT($B$1:$B$51)),1)