I started to build my CNC router Yesterday

I am making my router table out of steel. I got the basic table welded
up yesterday. The Gantry will move and the Gantry will be designed
from aluminum. The gantry will be approxinately 36" long. Where would
be the best place to purchase a 1/2 x 48" lead screw with about 8
threads per inch, as well as couple of rails 40" long and a couple 36"
long. I am looking for a place to purchase these at a decent price.