UHHH... with a few exceptions, the places that would carry latex are the same places that carry silicone. You gotta keep in mind that the materials used for most of the higher-end costumes is not hardware store kinda material. It's industry standard stuff and generally only found at special fx suppliers. You might find trial-size silicone at a few high-end arts and crafts or hobby stores. I've seen Smooth-On trial-size at Pearl Art Supplies. But you're going to need a lot of silicone for molding a set of armor and silicone is a lot more expensive than latex. And if you have no experience with silicone, well, that's a whole other conversation. You need to decide whether to go with platinum based or tin based silicone. Some of the platinum based are 1:1 ratio... that makes things easier. If it's not 1:1, you're going to need a gram scale and MAKE SURE you know how to do all your tare weights before dumping a bunch of silicone in and mixing (remember to account for the weight of the empty mixing containers as well)... If your ratio isn't accurate or you don't mix it properly, it won't cure properly. Tin based silicone really needs to be degassed too. This prevents air bubbles... you can work around this by slowly pouring from several feet above your sculpt, allowing the air bubbles to work themselves out and burst before they get to the original. But that's not likely to get them all out.
Maybe budget isn't as much of a concern for you as it is to most of us, and if not, that's awesome. But if it is, I'd recommend ordering some liquid latex from a supplier such as Burman Foam, Silpak or Monster Makers and go that route. It will be much cheaper and an awful lot easier. Either way, good luck and keep us posted with your progress.