like i said i've not done anything on walls yet, i'd rather refine the technique to a stage where the underspray is at the very least not noticeable - but when i get round to doing some more on paper i'll be sure to grab some pics
narate - fast-drying matte sprays are preferable, i agree the best thing to do is ask at an art store, they'll be able to direct you to the best option, though I have heard bog-standard Halfords spray works just as well..
oh, and if you haven't worked with spray-adhesive before, be careful - it's some seriously nasty shit. if you're spraying while holding the stencil make sure you're wearnig gloves and if you're not working outdoors a facemask would probably be a good idea (inhale enough of the stuff and it coats your lungs...)