Does it have to be downtown? I live in between Goosefoot and Fine Wine Brokers and that's where I would recommend you go.
Fine Wine Brokers is very close to Goosefoot (less than 5 minutes by car, 10 or so minutes walking, and close to the Western El station), and they are excellent. Also, they tend to keep up with the local byo restaurants and I suspect they have already provided great help to many people dining at Goosefoot (they helped me). They have a terrific selection of wines, including some already chilled.
If you are taking a taxi to the restaurant, you could always call FWB in advance to arrange for the wines to be pre-selected (with some guidance) so that you don't need to keep your cab waiting too long.