Yes, I see other people's amazing MOC's all the time, and think, "man, I could totally do that if I had enough bricks." The problem is, I don't really know where to look for a whole bunch of LEGO bricks that aren't in set form. I would really like the pile to include normal and Technic bricks, as well as some specialized parts such as gears, transmission pieces, wheels and tank tracks, but don't need any minifigure parts.
MOC-ers use BrickLink.com, the Unofficial Online LEGO Marketplace. It is similar to eBay in the sense that buyers and sellers are from all over the world, but BrickLink is only for LEGO. All elements are neatly organized into categories, and every LEGO element ever made is catalogued. Link to BrickLink Catalog Page: http://www.bricklink.com/catalogTree.asp?itemType=P
You can build up your Wanted List, then purchase the pieces you need from various sellers. (By building a Wanted List you can find sellers who have most of the items you need, thus reducing shipping fees.) Most BrickLink sellers have very reasonable prices and excellent service - they are LEGO fans themselves. Just make sure you check their feedback and terms before purchasing.
BrickLink is a MOC-er's heaven. :)