I need a source for

-1.75" Square chromoly w/ .120 and .188 wall
-3"x4" Rectangle chromoly w/ .188 wall

Anyone know of somewhere online I can order?

http://www.metalexpress.net has the 1.75" square 4140 Annealed chromoly on their web site but doesn't show a wall thickness.

http://www.industrialmetalsales.com
http://www.onlinemetals.com

may have what you need but just not list it on their web site. I have bought from the first two places and can recommend them. I dealt with the Baltimore, MD location of metalexpress.net