When you first start off, many times the spare parts are what you get off junker machines in the street, what your family pawns off on you, or something you pickup in a bargain bin. Then you start getting more and more parts, going to auctions, picking up old machines from work or school, and it adds up. A lot. At one point, I had enough parts to fill a 2 car garage. Well, with enough walking room, but I'm pretty skinny.

Basically it takes baby steps and it snowballs. Others may have different backgrounds, though they will probably be similar.

As for my first mod, it was actually integrating a NES into an aquarium stand and installing speakers. That was over 10 years ago now I think about it.
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MacBook 2GHz Intel Core 2 Duo w/ 2GB DDR2 RAM & 120GB SATA 5400RPM HDD
Canon Rebel XTI
Google Cr-48 Beta Laptop