It is a type of sturdy poster board, which has regular poster board on the outside and a core of lightweight, compacted foam in the middle. This makes a sturdy presentation board. It's great for kids' school projects, signs, any type of display, really. It's also used in framing.
Signarama has custom signs that one can buy. The signs are made from either aluminum, PVC, plastic, or FoamCore, depending on what one would like. The prices of these signs range from $56 to $409, depending on the size, material, and lettering.
You can purchase squares of cork boards at places like Lowe's and Home Depot. You can make a fake one with a thick card board and cover it with thin batting if you want, then some people do a criss-cross pattern of ribbon accross the front so you can slip things under them and not have to use as many tacks. My daughter's have gotten several of these home made ones over the years. It seems like they also taked down the ribbon where it crosses the ribbon going the other way (and it was quilted). As far as cost, I would go in your garage/attic and see if you can find a sturdy piece of cardboard. If not you can buy a piece of foamcore board (Sold at Micheals or any craft/art store and we'll assume you're going for a really big one. Good luck.
Welcome signs can be made in different materials and can be positioned in various places inside and outside. One can visit a shop that sells housewares to find doormats with the welcome sign, etc. If one wants something personalized, then one should visit a sign making company.