First of all, shout out to Reno. I've been to that waterpark more times than I can count.
For the paint, don't use Valspar. That stuff is terrible, low in pigment, fades, and runs like shit. I use Belton Molotow Premium and ironlak brands of paint. Buy paint off of artprimo.com. it's a specialized graffiti store and they carry everything you're going to need. Look into low pressure cans. You don't want spray going under the stencil.
As for what material to use for the stencil itself, try and stay away from cardbord as much as you can. It's difficult to get clean, crisp lines, high detail, etc. Use a medium thickness posterboard, manilla, etc. Stapels and Office Depot should have what youre looking for.
When you go out and paint, wear a face mask, all black or dark colors, carry a handbag to keep the cans in for fast access as opposed to a backpack. Shake the cans in an area close enough to the bank but far enough away that people wont hear. I put magnets on the bottom of my cans to keep the rattle balls in place and to keep my noise down when I go out.
Be keen, smart, quiet, and swift. Get in and get out asap (duh). Take pictures of your shit so we can see your work!
Hope this can help and good luck man!
"The female situation in Tahoe is like parking spots... All the good ones are taken, and all the other ones are either handicapped or its illegal to park." - K.C. Deane.
"Freeskiing is just what it says, free, you can do whatever you want."-Skierman.
RIP Shane McConkey and CR Johnson