Buy off the net... particularly have a look at red balls
www.redballs.com.
I recieved my reverend coat from them just yesterday and the only way i can describe it is marvellous - floor length full weight black canvas.
Theres no concievable way of finding a cheap leather trenchcoat, you may try ebay, ive got my eyes on a certain white leather one on there that goes for decent prices from its store, but your best real option for anything halfway decent is to buy online, particularly given how many places will cut to your exact measurements and tailor make it cheaper than youd buy one off the rack for in sydney.
Also, forget gallery serpentine. Dont even bother.