There's better and worse at Radio Shack. This "silver acrylic" goop for $23 ($16.50 from others) is fairly heavy-duty, but twitchy to handle -- shake it all the time, dry it under a light bulb for 20 min., sometimes use a mask(ing tape) to constrain flow. But, excellent characteristics, such as resistance < 0.017 ohms/mil^2. I've put 10 amps through it in a car.
The 'Shack also has this $10 "silver paint", which is great for fabric/paper, but not much else. Not heavy duty, twitchy to use, short shelf life, but it washes out; fun for kids.
Neither of these is really a "pen" though, nor are they fast-drying, which makes this KS project look interesting.