Will big box home supply stores cut Zinc-Plated Slotted Angle pieces to size for customers?
They may. 

But if not, you can do it with a reciprocating saw using a metal blade (but watch out; some are only rated for copper or brass) or a manual hack saw. The steel in these angles is pretty mild (soft) and is cut with just a little effort.

The most important thing is to firmly brace the metal before trying to cut.  If you have a vise, that is ideal. If not, you can place the metal over a piece of 2x4 so that both legs are sitting against wood. Clamp the steel to the wood. Firmly attach the 2x4 to some rigid item, screwing or nailing it on temporarily if necessary. 

Cut slowly. If the metal binds, you can use a little light oil to lubricate the cut. Each cut will only take a minute or two.