in my opinion, an eye for bonsai canidates is aquired over time, and makes no difference between whether it is in a pot on a shelf or in the ground. when I started, i bought several trees, and collected several trees, which, for all intents and purposes, could have neen identical to the nursery stock, yet they were all crap. now, I can sort of plan ahead as to what style, size, etc. I am just a novice but I honestly think the only thing about nursery stock is convenience. but think back to ancient times, the literati and bonsai entheusiasts couldn't just go down the street and buy a plant. I think those pre bonsai as mentioned are fine, but it feels sort of like a paint-by-number, rather than actualy designing the work yourself, you are just adding to what has already been done. for me, I get as much enjoyment out of finding trees and waiting to transplant (at least it is something to look foreward to) as i do styling the trees. now, I live in a large stand of trees, in a microclimate very similar to temperate china/Japan. Most traditional trees would grow well here, and many local trees lend themselvesa well to bonsai, although I have not taken advantage of this like I should, but for those in citys/towns or where collecting is illegal or impractical, Nursery stock is a good alternative. but back to my point, I don't think that you have to, or really should, start with nursery stock to get an eye for it. look at natural trees around you, look how the best pieces of that species are styled, and then find one that could be trained similarly. Of course, I also am a fanatic over tropicals, and not just for bonsai, so nurserys do have their place. as someone said above, just get a shovel, some pots, and know the best time of the year (and permission to collect, if neccesary!)
But thats just my opinion. you know what they say, Different strokes for different blokes.