Well that's something you just build using your imagination while using the source material, lots of trial and error involved. Once someone builds something like that, they either give others and idea on how/what pieces they used to build it so they can build their own; or don't bother to do so at all. Kind of like anyone who makes custom Transformer figures, oftentimes the customizer will say what figures were used in making the kitbash, and for those that don't it's not too hard to figure out what was used on TF customs (not a lot of parts compared to something like LEGO).
As for the pieces, yes they do exist, cutting and splicing together brick pieces is a BIG no-no in the LEGO community. You have to use existing LEGO pieces otherwise, it's not a LEGO build. Builders usually have the pieces already from LEGO sets that they own when they start working on something like this. Pieces can be bought separately through LEGO's website, but they do not have a big selection, very limited in fact. There's a resource/database site that shows all existing parts. The main feature of the site is their "store", which I linked you to a few months back when discussing the toyline expansion/setting up a store. When you click on say a 1x1 brick it will list all the colors that have been released to date, you can click on the particular color you are looking for then it will give you a listing of all the users who are selling that 1x1 brick in that particular color.
--SoS
