Check out vinylnation.net for loads of information about Vinylmation. The selection will tend to be like pins. Most stores will have a small selection, some have alot. The small selections tend to be 2-3 series of the blind boxes. D-Street at DtD tends to have the biggest selection, as well as getting them before the other stores do. Most of the other locations won't carry a selection go the 9" ones, D-Street carries pretty much all of them. They also sell one of a kind pieces made by artists. AK doesn't seem to carry a large selection anywhere, Epcot tends to carry them at Mouse Gears, DHS is Villains in Vogue and the first store on the left as you enter the park, MK usually the Emporium, or the Frontierland Pin Store. The online selection tends to have a really good selection, and carries some exclusives. Odds are if the stores carry it, they will as well. The only exception being some of the limited edition pieces.
Trading is supposed to be one trade per "black box" and one trade per clear box per person per store per day. Now there is no way to really police that, shift changes, simply getting a different CM, etc. Most stores that sell vinyls will have at least the 3 vinyl clear box, and the stores that sell more of them will usually have the black box as well. Now some CMs will be strict about the rules. Technically you are supposed to pick a number on the black box, and that is what you get. Other CMs will offer up hints, or let you pick a second number if the first one wasn't something you wanted. A good rule of thumb, is to trade black box first, and if you don't like it, you can then trade clear box. There is no real set location to trade vinyls like they have for pins. Some vinyl traders will hang out at the pin tables, or trade at the pin/vinyl trade nights.