or could i just get it at a store? (i.e walmart, home depot, etc)

hw many plants u thinking of growing or how big u thinking of making the box?
u can pick up fans from alot of diy stores but im in uk so we dnt have walmart and that, but like i say unless ur doing a resonably big grow u wont need a squirel cage fan just a extract fan.
spacing is a tough one, my pots are just next to each other as close as i can get them.
just get em as close to each other as pos and if they need more space them move them apart,

I only want to grow three max.

Thanks Littlewing u have been a ton of help.