All of those places would have it, or you can get it online of course. Probably wherever Cricut products are sold. Are you just doing one, or are you thinking of doing a few of the same? I think the vinyl would work well, but you'd have to cut out more than 1 if you were thinking of doing different things around the room. The vinyl is easy to remove from the walls (or wherever) but it's not reuseable.
or if there's a sign making place near you, they sometimes have scraps that they give away. I keep forgetting to check at the one by me.
I bought mine online, much cheaper than local retail.
you're going to want transfer paper too, if you're using it to put on walls.
Goodness, I have no idea what I'm doing. I'm going to be making a border by stenciling elephants all the way around the room, so I'll be making a couple, but using them lots if possible. Would cardstock work, I wonder? Gwen, what is the transfer paper for? Going to go google this and see if I can find instructions, sigh.
the transfer paper is to keep the vinyl exactly where you want it. it's like painter's tape, it doesn't stick for good. I wouldn't use cardstock for what you're doing. but I'm not sure I'd use vinyl either, as you don't want to take off any paint when you move it (I'm assuming you mean the vinyl that sticks). Cricut has a great message board on their site, you could always search it. I bet you'll find some good stencil advice there.
I think Vinyl would work well for what you're doing if you use the negative space of your cut - but not if you need to use it over and over. I don't think cardstock would work well at all. I imagine that some paint would get underneath it, you know? If you're just doing a simple design you don't need transfer tape. I didn't use ANY transfer tape at all when I did Miles' cars and trucks in his playroom. If I were doing words or something I would use it for sure.
I think you could probably cut a thin plastic similar to what a premade border stencil would be made out of. not sure where you could get a plastic like that, though.
eta: I might have seen a sheet of plastic like that in the stencil borders section before for people to design/cut their own border...maybe i am making that up, but I swear it exists