The majority of the foam at fabric stores is for upholstery projects, and therefore the only real differences you'll notice in what such stores typically have will be in the thickness and density of the foam.
A denser foam would be better for armor as it will appear more solid and flop around less (as well as flake less) than a lower density foam. How thick the foam is will depend on personal choice more than anything.
Cutting polyurethane foam can be somewhat obnoxious, and sometimes scissors/shears are not the best way to go. When I worked at a fabric store, we actually had a small hand saw we used to cut the denser/thicker pieces, but a box cutter or other similiar instruments should also do the trick while giving you more control than a scissors would provide.
How are you planning to color the foam? If you're planning to use paint it, be careful to test any paint out before you use it on the actual armor pieces, as some paints (spray paints in particular) will basically eat polyurethane foam. Acrylic paints shouldn't cause any issues, however. You could, of course, also cover the foam with fabric or vinyl.
Hope that helps somewhat. If you have other questions about the foam, feel free to ask. ^.^