Hey man. I try not to search ebay as much as possible as there is a great deal of fakes going around to be honest. I am a member of a facebook group called Centralemaxx, send me a pm with ur facebook details of you would like me to add you. That is where I get most of my parts. Over the last 3-4 years I have made a lot of friends in the hobby with a similar interest in rare parts as me. I live in Australia, so am very isolated when it comes to this hobby, but through the hobby have made very close friends in the states and all of europe...
Everything I know about polishing I learnt from a friend of mine on the UE forums. Sweetride4Me was one of the 2 guys behind the sweetstrobe braces for the maxx trucks, and is a great guy. He loves to hand polish, and after several of us asked how it is done, he put aside a weekend and documented a how to, as well as post up some youtube videos also. Check it, as it is a good way to get started:
http://www.unlimitedengineering.com/...ghlight=polish
Regarding what grit sandpaper to use, it kind of comes down to what you are working with, & how scratched up it is... If you say were working on a new part, without any abuse, then I would start with 1000 grit, then onto 1200, 1500 and finish off with 2000. Once you have gone over the part with 2000, you can then polish using Meguirs Metal Polish to get rid of all the small scratches and bring out that mirror finish. It is very much you get what you put into it. The more you sand, the better the result. If you are working on used parts, then it will save you a lot of heart ache to hit it with 400 grit 1st, then 800, then 1000 & so on...