I would recommend walking through landscape nurseries and looking for the first couple inches of trunk and the first three branches. All else is changeable by bonsai practices. Also, don't fall in love with cheap marked-down plants at the end of the growing season because they are on their last legs, that's why they want to sell them cheap. The best time to find good stock in a landscape nursery is when they have a billion plants at the beginning of the season. They are healthy, they are right there in your face to rotate the pot, examine extensively and ponder a pathway to show quality, and compare with everything else available as to species, variety, size, price, and so on. It's a fun adventure and you can spend delightful hours shopping and once you learn how to shop specifically for bonsai stock, you'll be a lot happier with your results. Volume is the name of the game: hands-on learning to style, and becoming able to look at a so-so plant and put it back down and walk to the next one. You have graduated when you can stroll into a nursery with money burning a hole in your pocket and leave with nothing.