Each year i start a few hundred seeds and cuttings, keep 10-20 of each variety that look promising for leaf size, colour, or other trait, sell off or give away the others and continue the best ones. Along the way i'll keep whittling down and selling off to one or two particularly good trees out of the original hundreds. Eventually i'll hopefully have some nice trees like Shibui has posted.
I also have some stumps that have been cut down drastically, and now i'm having to cut them back even harder and carve deadwood into them to make them a bit more acceptable. I only do this with Ginkgo as they are incredibly slow to grow thickness in the trunk, but even Ginkgo i start from seed and cutting to have some nice un-scarred ones in the future.
Don't let the length of time deter you. If you are going to keep bonsai, 15 years doesn't matter too much, those years will pass anyway, and i guarantee you'll be more attached to something you grew from a seed than a piece of material you purchased already as a more mature plant.
I'm not saying don't buy more advanced bits of material, it's got benefits of its own and i certainly do it, but i'm saying growing from seedlings is hugely rewarding and well worth persevering with.
