I mostly buy from fellow modellers, like Andrew Herbert aka Herberta on Armorama, or on "the evil ebay". I pay with money orders, but must remember to hide the receipt in my desk at work, almost got caught! Or I pay by cash. I usually have the kits sent to my office and then take them home in an empty copy paper box. I had 3 kits on the bookshelf next to my desk at work for a month and she didn't notice them during her frequent visits mixed into the stack of folders, papers and books etc. pretty clever eh?
Usually I wait until my wife is at her cross stitch club on tuesdays and my 5 year old is brushing his teeth and changing into jammys. He's sharp when it comes to tanks and airplanes and can spot a C-130 at 4 miles! Can't keep a secret either.
I sneak it into the garage and add it to the stacks on the top shelf, which is strategically re-organized once a month. This is important so that she doesn't recognize the new additions. I also have a stack on my bench that I move around occasionally. I also use the same strategy that Rob does, leave them in plain sight on the table until she tells me to put them away. If we are in Hobby Lobby together and I find one on sale, I tell her about how much it is on line or at the other stores and she then can justify the purchase since its "on sale".
So I usually employ the following tactics: Hide and Sneak, Mix with Mess, Buy on Sale, and Re-arrange the Stack. All are a must for the Modelholic. If these fail, try throwing yourself on her lap sobbing "its a disease and i can't help myself!". Havn't tried this yet, but if anyone does, let me know how it turns out, if you're still able that is!