Heyyy
Although I don't build strats personally, I have a few friends who do. And know of some people... I guess.
The most common way they do it is buying parts bit by bit because they have their own mm... image of what kinda guitar/strat they want. A certain pickguard, body, neck bolts, etc etc...
And usually from eBay too. I'm not promoting eBay in anyway but I'm just saying that's the place where they usually get their parts. Because if it's the same seller selling the parts you want, I guess you can save on shipping. (Which is mostly the case anyway.)