Try the Gap boys department. They usually have navy blue blazers, and the larger boys sizes usually fit average to petite women. The sleeves are usually shorter, so you'll have to roll them to make a 3/4 sleeve. Not to mention the boys clothes are cheaper than womens.
Look out for Kate Moss For Topshop's upcoming (or is it latest? I'm not too sure) collection. Someone on TPF posted the pics for the clothes from that particular collection and there was a navy blazer if I remember correctly.