....And still; in all the debate about price, availability, and the uncertainty of buying off the Internet, even from good sources, little mention is made of the common garden variety nursery. It seems there are some that avoid this market like the plague when in fact anyone with the right knowledge and perseverance to look through rows of trees can still find something as good as many of the pre-bonsai purchased at ten times the price. You may be limited in the species available but if you think there is nothing to be had in the common nursery you are sadly wrong.
Having said that I must point out that the so called Big Box stores usually have material that is here today and gone tomorrow and of little use in bonsai for the most part, but still you can occasionally find something of worth in one of them. The best places to look is in the Mom and Pop nurseries where they take care of their material and may keep it for years. Often you can find some nice things at bargain prices, if you know what you are looking for and don't mind getting your hands dirty and your nails full of crud.
It is also true that nursery trees take special care, mostly to regrow a root system suitable for bonsai; but so do Yamadori. As to the question; is there a shortage of pre-bonsai material? The answer is it depends on what you are looking for, how much you are willing to pay, and the kind of experience you have in identifying it and the ability to work with it. The fact remains that unless you have the time, physical ability, the geographic access, and the experience to collect it, you are left with one choice; pay for it. The availability of world class material will not be available to you unless you are willing to pay world class prices for it.
During trouble financial times the general nursery may yield to you some surprises for next to nothing in some cases. Word of warning: This stuff will not jump out at you and scream Bonsai here, you will have to struggle through a lot of trees to find something worthy.