A good butchers is obviously going to give you the best results but you can get a lot of larger cuts of meat from supermarkets if you ask on the butcher counter. Slabs of ribs, whole briskets, pork shoulders, etc I've managed to get from my local Tesco. Actually, for a beginner I'd recommend going for "beef clod" rather than brisket. It's essentially beef shoulder and is often sold as 'stewing steak'. Ask the butcher if they've got a hunk of it in the fridge behind the counter. Treat it like brisket in terms of cooking...however, it has a lot more connective tissues and fat that'll break down and keep it super moist during the course of low and slow cooking.